Proof that angles in the same segment are equal

Builds on Inscribed angle theorem Concyclic points Oriented angle Angle at a point Circle Line Line segment Determinant Sine is 2π-periodic

Proof

We compare both angles with the angle at the centre.

The points are concyclic in $\mathbb{R}^2$ , so they lie on a circle with a centre $O$ and a radius $r \geq 0$ . Since $A \neq B$ , we have $r > 0$ .
By the inscribed angle theorem with $V = C$ and with $V = D$ , we get $2\,\measuredangle ACB = \measuredangle AOB = 2\,\measuredangle ADB$ . This is the first claim.
Let $\theta_C$ and $\theta_D$ be real numbers that represent the oriented angles $\measuredangle ACB$ and $\measuredangle ADB$ . By step 2, $2\theta_C - 2\theta_D \in 2\pi\mathbb{Z}$ , so $\theta_C - \theta_D \in \pi\mathbb{Z}$ .
Hence either $\theta_C \equiv \theta_D$ or $\theta_C \equiv \theta_D + \pi$ modulo $2\pi$ .
For the second part, put $f(X) = \det(B - A, X - A)$ for $X \in \mathbb{R}^2$ , with the determinant. Since $B - A \neq 0$ , $f(X) = 0$ if and only if $X$ is on the line $AB$ .
The points of the line $AB$ are $A + t(B - A)$ . Such a point is on the circle when $\norm{A - O + t(B - A)}^2 = r^2$ . This is a quadratic equation in $t$ with the leading coefficient $\norm{B - A}^2 > 0$ .
So the line meets the circle in at most two points, namely $A$ and $B$ . Hence $C$ and $D$ are not on the line $AB$ , and $f(C) \neq 0$ , $f(D) \neq 0$ .
Let $X$ , $Y$ be points off the line $AB$ . They are on the same side of it when the segment $XY$ misses the line. The determinant is linear in its second column, so $f((1 - t)X + tY) = (1 - t)f(X) + t f(Y)$ .
If $f(X)$ and $f(Y)$ have the same sign, then $(1 - t)f(X) + tf(Y)$ has that sign for $0 \leq t \leq 1$ . So the segment misses the line.
If they have opposite signs, then $t = \frac{f(X)}{f(X) - f(Y)}$ lies in $(0, 1)$ and makes the value zero. So the segment meets the line. Thus $X$ , $Y$ are on the same side if and only if $f(X) f(Y) > 0$ .
By the definition of the oriented angle, $\sin\theta_C = \frac{\det(A - C, B - C)}{\norm{A - C}\norm{B - C}}$ .
Since $B - C = (B - A) + (A - C)$ , we get $\det(A - C, B - C) = \det(A - C, B - A) = \det(B - A, C - A) = f(C)$ . So $\sin\theta_C$ has the sign of $f(C)$ . In the same way, $\sin\theta_D$ has the sign of $f(D)$ .
The angle $\angle ACB$ is the unique number in $[0, \pi]$ whose cosine is $\cos\theta_C$ , since both equal $\frac{(A - C) \cdot (B - C)}{\norm{A - C}\norm{B - C}}$ . The same holds for $\angle ADB$ and $\theta_D$ .
Suppose that $C$ and $D$ are on the same side. Then $\sin\theta_C$ and $\sin\theta_D$ have the same sign by steps 10 and 12. Since sine has period $2\pi$ and $\sin(\theta + \pi) = -\sin\theta$ , the case $\theta_C \equiv \theta_D + \pi$ is excluded.
So $\theta_C \equiv \theta_D$ , hence $\cos\theta_C = \cos\theta_D$ . By step 13, $\angle ACB = \angle ADB$ .
Suppose that $C$ and $D$ are on opposite sides. Then the sines have opposite signs, so $\theta_C \not\equiv \theta_D$ . By step 4, $\theta_C \equiv \theta_D + \pi$ , so $\cos\theta_C = -\cos\theta_D$ .
The number $\pi - \angle ADB$ lies in $[0, \pi]$ , and its cosine is $-\cos\angle ADB = -\cos\theta_D = \cos\theta_C$ . By step 13, $\angle ACB = \pi - \angle ADB$ , that is, $\angle ACB + \angle ADB = \pi$ .

Idea

Both inscribed angles are half the central angle, but only up to a half turn. The sign of the sine of an oriented angle tells on which side of $AB$ the vertex lies. On the same side the half turn cannot appear, and on opposite sides it must appear.

Reading path

To understand this page from the ground up, read these 299 pages in order. The foundations come first. The path takes the proofs Proof of the Cauchy–Hadamard theorem, Proof of the derivative of cosine by term-by-term differentiation, Proof of the Weierstrass M-test, Proof of the limit of a sum of sequences, Proof of Rolle's theorem, Proof of the approximation property of the supremum, Proof that every sequence has a monotone subsequence, by peaks, Proof of the formula for the derivative of a polynomial, Proof of the commutativity of addition by induction, Proof of term-by-term differentiation, Proof of the epsilon-delta form of continuity, Proof of the power rule by induction, Proof that absolutely convergent series converge, Proof of the sum of a finite geometric series, Proof that convergent sequences are Cauchy, Proof that a complex number times its conjugate is the square of its modulus, Proof that there is a natural number n with 1/n below any positive epsilon, Proof of the chain rule with an auxiliary difference quotient, Proof of the Cauchy criterion for series, Proof of the Cauchy criterion for sequences by the Bolzano–Weierstrass theorem, Proof that cos pi = -1 and sin pi = 0, Proof of the equality of complex numbers, Proof that the reals form an ordered field, with the Cauchy sequence construction, Proof of the root test, Proof of the double negation law by a truth table, Proof that power series converge uniformly on smaller intervals inside the radius of convergence, Proof of the extreme value theorem by the Bolzano–Weierstrass theorem, Proof that subsequences of a convergent sequence converge to the same limit, Proof of the uniqueness of limits, Proof of strong induction from ordinary induction, Proof that power series are differentiable inside the radius of convergence, Proof of the limit of a product of sequences, Proof that uniform convergence of derivatives allows term-by-term differentiation, Proof that the absolute value is multiplicative, Proof of the squeeze theorem for sequences, Proof of the existence of square roots of nonnegative reals, Proof of Pascal's rule from the recursive definition, Proof that i squared is minus one, Proof of the linearity of differentiation, Proof of the angle addition formulas by differentiation, Proof of the sequential characterization of limits, Proof that Cauchy sequences are bounded, Proof of the triangle inequality for real numbers, Proof of the epsilon-N definition of the limit of a sequence, Proof that the modulus is multiplicative, Proof of the comparison test, Proof of the existence of the infimum by negation, Proof of the continuity of the inverse of a strictly monotone continuous function, Proof of Fermat's theorem on stationary points, Proof that limits preserve non-strict inequalities, Proof of the factorial formula for binomial coefficients by induction, Proof that a function with positive derivative is strictly increasing, Proof of the product of exponentials by the Cauchy product, Proof that the complex numbers form a field, Proof of the intermediate value theorem by the supremum, Proof of the binomial theorem by induction, Proof that exponentials dominate powers in sequences, by the ratio of consecutive terms, Proof that differentiable functions are continuous, Proof that one-sided limits of a monotone function exist, Proof that sine and cosine are 2π-periodic with smallest period 2π, Proof of the epsilon-delta form of the limit of a function, Proof of the derivative of sine by term-by-term differentiation, Proof of the well-ordering principle from strong induction, Proof of the Bolzano–Weierstrass theorem by a monotone subsequence, Proof of the Archimedean property in an Archimedean ordered field, Proof of the existence and uniqueness of the principal value of the argument, Proof of the polar form of a complex number, Proof that a composition of continuous functions is continuous, Proof of the Pythagorean trigonometric identity by differentiation, Proof that a decreasing sequence bounded below converges, Proof of the term test, Proof that the reals are Archimedean from the completeness axiom, Proof of the ratio test, Proof that convergent sequences are bounded, Proof of the product rule, Proof of the monotone convergence theorem for sequences, Proof that continuity is equivalent to sequential continuity, Proof that a function with zero derivative on an open interval is constant, Proof of the boundedness theorem by Bolzano–Weierstrass, Proof of Euler's formula by power series, Proof that a series of nonnegative terms converges iff its partial sums are bounded, Proof of the mean value theorem from Rolle's theorem, Proof that the Cauchy product of absolutely convergent series converges absolutely to the product, Proof of the inscribed angle theorem by complex numbers, Proof of the constant multiple rule.

Angle at a point concept
Binomial coefficient definition
Circle concept
Complex conjugate concept
Complex numbers object
Complex numbers as pairs of reals concept
Concyclic points concept
Construction of the reals from Cauchy sequences concept
Definition by recursion concept
Determinant concept
Dot product concept
Element definition
Even and odd functions definition
Factorial definition
Group concept
Imaginary unit object
Integers object
Limit point of a set definition
Line concept
Line segment concept
Local maximum definition
Mathematical induction concept
Natural numbers object
Ordered field definition
Oriented angle concept
Peano axioms concept
Periodic function definition
Proposition concept
Rational numbers object
Real numbers object
Sequence definition
Set bounded above definition
Strictly monotone function definition
Proof of Pascal's rule from the recursive definition proof
Complex exponential definition
Power series in the complex plane definition
Real and imaginary parts definition
Proof that i squared is minus one proof
Set definition
Ring definition
Ring of integers statement
Limit of a function definition
Local minimum definition
Archimedean ordered field definition
Successor function definition
Conjunction definition
Disjunction definition
Implication definition
Negation definition
Predicate definition
The reals form a field statement
Real sequence definition
Subsequence definition
Infimum definition
Supremum definition
Pascal's rule statement
Complex conjugate definition
Ordered pair definition
Commutative ring definition
Unit of a ring definition
i squared is minus one statement
Continuity at a point definition
Derivative definition
One-sided limit definition
Biconditional definition
Truth table concept
Addition of natural numbers definition
Modus ponens concept
Existential quantifier definition
Universal quantifier definition
Proof of the Archimedean property in an Archimedean ordered field proof
The rationals form a field statement
Bounded sequence definition
Cauchy sequence definition
Geometric sequence definition
Limit of a sequence definition
Monotone sequence definition
Sequence tending to infinity definition
Series definition
The successor function is injective statement
Zero is not a successor statement
Completeness axiom statement
Cartesian product definition
Field definition
Proof of the factorial formula for binomial coefficients by induction proof
Proof of the equality of complex numbers proof
Continuous function definition
Differentiable function definition
One-sided derivative definition
Subset definition
Mathematical proof concept
Archimedean property statement
Uniqueness quantifier definition
Proof of the commutativity of addition by induction proof
Propositional logic concept
Order on the natural numbers definition
Limit superior definition
Proof of the sum of a finite geometric series proof
The rationals form an ordered field statement
Convergent sequence definition
Pointwise convergence definition
Sequential continuity definition
The sequence r^n tends to infinity for r > 1 example
Cauchy product definition
Partial sum definition
Power series definition
Equality of complex numbers statement
Binary relation definition
Proof that a composition of continuous functions is continuous proof
Factorial formula for binomial coefficients statement
Tautology definition
Order on the reals definition
Commutativity of addition statement
Sum of a finite geometric series statement
Proof of strong induction from ordinary induction proof
Uniform convergence definition
Radius of convergence definition
Convergent series definition
Sine and cosine definition
Antisymmetric relation definition
Reflexive relation definition
Transitive relation definition
Proof of the binomial theorem by induction proof
Interval definition
Proof that the complex numbers form a field proof
Function definition
Logical equivalence definition
Uniform convergence on a set definition
Strong induction statement
Sum of a series definition
Series of functions definition
Absolute convergence definition
Preorder definition
Binomial theorem statement
Proof of the epsilon-delta form of continuity proof
Proof of the epsilon-delta form of the limit of a function proof
The complex numbers form a field statement
Proof of the well-ordering principle from strong induction proof
Proof of the double negation law by a truth table proof
Partial order definition
Monotone function definition
Epsilon-delta definition of continuity statement
Epsilon-delta definition of a limit statement
Double negation law statement
Well-ordering principle statement
Total order definition
Composition of continuous functions is continuous statement
Proof by contradiction concept
Proof that every sequence has a monotone subsequence, by peaks proof
Proof that the reals form an ordered field, with the Cauchy sequence construction proof
Every sequence has a monotone subsequence statement
The reals form an ordered field statement
Absolute value definition
Proof of the approximation property of the supremum proof
Proof of the existence of square roots of nonnegative reals proof
Proof of the existence of the infimum by negation proof
Proof of the triangle inequality for real numbers proof
Proof that the absolute value is multiplicative proof
Proof of the epsilon-N definition of the limit of a sequence proof
Existence of square roots of nonnegative reals statement
Approximation property of the supremum statement
Existence of the infimum statement
Triangle inequality statement
Absolute value is multiplicative statement
Square root definition
Epsilon-N definition of the limit of a sequence statement
Proof that one-sided limits of a monotone function exist proof
Proof of the intermediate value theorem by the supremum proof
Proof that the reals are Archimedean from the completeness axiom proof
Proof of the limit of a sum of sequences proof
Proof of the squeeze theorem for sequences proof
Proof of the uniqueness of limits proof
Proof that limits preserve non-strict inequalities proof
Modulus of a complex number definition
Proof that differentiable functions are continuous proof
Intermediate value theorem statement
Proof that convergent sequences are bounded proof
Proof that convergent sequences are Cauchy proof
The reals are Archimedean statement
The sequence 1/n tends to zero example
Proof that Cauchy sequences are bounded proof
Proof of the term test proof
Proof that subsequences of a convergent sequence converge to the same limit proof
Proof of the monotone convergence theorem for sequences proof
Proof that a complex number times its conjugate is the square of its modulus proof
Argument of a complex number definition
Differentiable functions are continuous statement
Limit of a sum of sequences statement
Uniqueness of limits statement
Limits preserve non-strict inequalities statement
Proof of the continuity of the inverse of a strictly monotone continuous function proof
Squeeze theorem statement
Proof that there is a natural number n with 1/n below any positive epsilon proof
Convergent sequences are bounded statement
Subsequences of a convergent sequence converge to the same limit statement
Convergent sequences are Cauchy statement
Cauchy sequences are bounded statement
Monotone convergence theorem for sequences statement
A complex number times its conjugate statement
Proof of the limit of a product of sequences proof
One-sided limits of a monotone function exist statement
There is a natural number n with 1/n below any positive epsilon statement
Term test statement
Proof that a series of nonnegative terms converges iff its partial sums are bounded proof
The sequence r^n tends to zero for |r| < 1 example
Proof that a decreasing sequence bounded below converges proof
Proof that the modulus is multiplicative proof
Proof of the sequential characterization of limits proof
Proof that continuity is equivalent to sequential continuity proof
Decreasing sequence bounded below converges statement
Limit of a product of sequences statement
Geometric series example
Continuity of the inverse of a strictly monotone continuous function statement
A series of nonnegative terms converges iff its partial sums are bounded statement
Modulus is multiplicative statement
Proof of Euler's formula by power series proof
Continuity is equivalent to sequential continuity statement
Proof that the Cauchy product of absolutely convergent series converges absolutely to the product proof
Proof of the root test proof
Sequential characterization of limits statement
Proof of the ratio test proof
Proof of the Cauchy–Hadamard theorem proof
Proof that exponentials dominate powers in sequences, by the ratio of consecutive terms proof
Proof of the Bolzano–Weierstrass theorem by a monotone subsequence proof
Proof of the comparison test proof
Euler's formula statement
Proof of the chain rule with an auxiliary difference quotient proof
Proof of the product rule proof
Proof of the linearity of differentiation proof
Comparison test statement
Cauchy product of absolutely convergent series statement
Bolzano–Weierstrass theorem statement
Exponentials dominate powers in sequences statement
Chain rule statement
Product rule statement
Linearity of differentiation statement
Proof of the product of exponentials by the Cauchy product proof
Ratio test statement
Root test statement
Proof of the boundedness theorem by Bolzano–Weierstrass proof
Proof of the Cauchy criterion for sequences by the Bolzano–Weierstrass theorem proof
Proof of the constant multiple rule proof
Product of exponentials statement
Proof of the power rule by induction proof
Boundedness theorem statement
Cauchy–Hadamard theorem statement
Cauchy criterion for sequences statement
Constant multiple rule statement
Power rule statement
Proof of the extreme value theorem by the Bolzano–Weierstrass theorem proof
Uniformly Cauchy sequence of functions definition
Proof of the Cauchy criterion for series proof
Extreme value theorem statement
Proof of the formula for the derivative of a polynomial proof
Proof of Fermat's theorem on stationary points proof
Cauchy criterion for series statement
Derivative of a polynomial statement
Proof that absolutely convergent series converge proof
Fermat's theorem on stationary points statement
Absolutely convergent series converge statement
Proof of Rolle's theorem proof
Rolle's theorem statement
Proof of the Weierstrass M-test proof
Proof of the mean value theorem from Rolle's theorem proof
Weierstrass M-test statement
Mean value theorem statement
Proof that power series converge uniformly on smaller intervals inside the radius of convergence proof
Proof that a function with zero derivative on an open interval is constant proof
Proof that uniform convergence of derivatives allows term-by-term differentiation proof
Power series converge uniformly on smaller discs inside the radius of convergence statement
Proof that a function with positive derivative is strictly increasing proof
A function with zero derivative is constant statement
Uniform convergence of derivatives allows term-by-term differentiation statement
A function with positive derivative is strictly increasing statement
Proof of term-by-term differentiation proof
Term-by-term differentiation statement
Proof that power series are differentiable inside the radius of convergence proof
Power series are differentiable inside the radius of convergence statement
Proof of the derivative of cosine by term-by-term differentiation proof
Proof of the derivative of sine by term-by-term differentiation proof
Derivative of cosine statement
Derivative of sine statement
Proof of the angle addition formulas by differentiation proof
Proof of the Pythagorean trigonometric identity by differentiation proof
pi object
Pythagorean trigonometric identity statement
Angle addition formulas statement
Arccosine definition
Proof that cos pi = -1 and sin pi = 0 proof
cos pi = -1 statement
Proof that sine and cosine are 2π-periodic with smallest period 2π proof
Sine is 2π-periodic statement
Proof of the existence and uniqueness of the principal value of the argument proof
Principal value of the argument statement
Proof of the polar form of a complex number proof
Polar form statement
Proof of the inscribed angle theorem by complex numbers proof
Inscribed angle theorem statement
Proof that angles in the same segment are equal proof

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Angles in the same segment are equal

Tag	`PRPWTE5WH6`
URI	`m2://PRPWTE5WH6`
Kind	proof
Writing	W0 stub
MSC	51M04
Proves	Angles in the same segment are equal
Uses	Inscribed angle theorem, Concyclic points, Oriented angle, Angle at a point, Circle, Line, Line segment, Determinant, Sine is 2π-periodic
Changed	4 November 2026

Proof that angles in the same segment are equal

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