Proof of Heron's formula from the law of cosines

Builds on Area of a triangle Triangle Euclidean distance Angle at a point Law of cosines Sine and cosine Pythagorean trigonometric identity

Proof

We compute $\sin \gamma$ from the sides and insert it in the formula $T = \frac{1}{2} ab \sin \gamma$ .

Let $a = d(B, C)$ , $b = d(C, A)$ and $c = d(A, B)$ , with the Euclidean distance $d$ . Let $\gamma = \angle ACB$ be the angle at $C$ , the angle between the sides of lengths $a$ and $b$ . The vertices are distinct, so $a, b > 0$ .
The area of the triangle is $T = \frac{1}{2} ab \sin \gamma$ .
By the law of cosines, $c^2 = a^2 + b^2 - 2ab \cos \gamma$ . So $\cos \gamma = \dfrac{a^2 + b^2 - c^2}{2ab}$ .
The sine is nonnegative on $[0, \pi]$ , and $\gamma \in [0, \pi]$ . By the identity $\sin^2 + \cos^2 = 1$ , $\sin \gamma = \sqrt{1 - \cos^2 \gamma}$ .
By step 3, $1 - \cos^2 \gamma = \dfrac{4a^2b^2 - (a^2 + b^2 - c^2)^2}{4a^2b^2}$ . Let $Q = 4a^2b^2 - (a^2 + b^2 - c^2)^2$ . Step 4 shows $Q \geq 0$ , and $\sin \gamma = \dfrac{\sqrt{Q}}{2ab}$ .
By steps 2 and 5, $T = \frac{1}{2} ab \cdot \dfrac{\sqrt{Q}}{2ab} = \dfrac{\sqrt{Q}}{4}$ .
As a difference of squares, $Q = (2ab - a^2 - b^2 + c^2)(2ab + a^2 + b^2 - c^2) = \left( c^2 - (a - b)^2 \right) \left( (a + b)^2 - c^2 \right)$ .
Two more differences of squares give $Q = (c - a + b)(c + a - b)(a + b - c)(a + b + c)$ .
Since $a + b + c = 2s$ , the four factors are $-a + b + c = 2(s - a)$ , $a - b + c = 2(s - b)$ , $a + b - c = 2(s - c)$ and $a + b + c = 2s$ .
So $Q = 16 \, s (s - a)(s - b)(s - c)$ .
By steps 6 and 10, $T = \dfrac{\sqrt{16 \, s(s - a)(s - b)(s - c)}}{4} = \sqrt{s (s - a)(s - b)(s - c)}$ .

Idea

Two sides and the angle between them give the area, and the law of cosines gives that angle from the three sides. The expression under the square root factors completely by differences of squares. Each factor is twice $s$ or twice $s$ minus a side.

Reading path

To understand this page from the ground up, read these 235 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 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 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 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 Pascal's rule from the recursive definition, Proof of the linearity of 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 of the comparison test, 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 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 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 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 law of cosines with the dot product, 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 that a series of nonnegative terms converges iff its partial sums are bounded, Proof of the mean value theorem from Rolle's theorem, Proof of the constant multiple rule.

Angle at a point concept
Binomial coefficient definition
Construction of the reals from Cauchy sequences concept
Definition by recursion concept
Dot product concept
Element definition
Euclidean distance concept
Factorial definition
Group concept
Integers object
Length of a vector concept
Limit point of a set definition
Local maximum definition
Mathematical induction concept
Natural numbers object
Ordered field definition
Peano axioms concept
Proposition concept
Rational numbers object
Real numbers object
Sequence definition
Set bounded above definition
Sine and cosine concept
Triangle concept
Proof of Pascal's rule from the recursive definition proof
Set definition
Ring definition
Proof of the law of cosines with the dot product proof
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
Area of a triangle concept
Pascal's rule statement
Ordered pair definition
Commutative ring definition
Continuity at a point definition
Derivative definition
One-sided limit definition
Law of cosines statement
Addition of natural numbers definition
Biconditional definition
Truth table concept
Modus ponens concept
Proof of the Archimedean property in an Archimedean ordered field proof
The rationals form a field statement
The successor function is injective statement
Zero is not a successor statement
Universal quantifier definition
Bounded sequence definition
Cauchy sequence definition
Geometric sequence definition
Limit of a sequence definition
Monotone sequence definition
Sequence tending to infinity definition
Series definition
Completeness axiom statement
Cartesian product definition
Proof of the factorial formula for binomial coefficients by induction proof
Continuous function definition
Differentiable function definition
One-sided derivative definition
Subset definition
Archimedean property statement
Proof of the commutativity of addition by induction proof
Mathematical proof concept
Order on the natural numbers definition
Propositional logic concept
The rationals form an ordered field statement
Limit superior definition
Proof of the sum of a finite geometric series proof
Convergent sequence definition
Pointwise convergence definition
Sequential continuity definition
The sequence r^n tends to infinity for r > 1 example
Partial sum definition
Power series definition
Binary relation definition
Proof that a composition of continuous functions is continuous proof
Factorial formula for binomial coefficients statement
Order on the reals definition
Commutativity of addition statement
Tautology definition
Proof of strong induction from ordinary induction proof
Uniform convergence definition
Sum of a finite geometric series statement
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
Uniform convergence on a set definition
Logical equivalence 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
Proof of the well-ordering principle from strong induction proof
Proof of the double negation law by a truth table proof
Partial order 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 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
Approximation property of the supremum statement
Triangle inequality statement
Absolute value is multiplicative statement
Epsilon-N definition of the limit of a sequence statement
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
Proof that differentiable functions are continuous proof
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 of the term test proof
Proof that Cauchy sequences are bounded proof
Proof of the monotone convergence theorem for sequences proof
Differentiable functions are continuous statement
Limit of a sum of sequences statement
Proof that there is a natural number n with 1/n below any positive epsilon proof
Uniqueness of limits statement
Limits preserve non-strict inequalities statement
Convergent sequences are bounded statement
Squeeze theorem statement
Convergent sequences are Cauchy statement
Cauchy sequences are bounded statement
Monotone convergence theorem for sequences statement
Proof of the limit of a product of sequences proof
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
Proof that a decreasing sequence bounded below converges proof
The sequence r^n tends to zero for |r| < 1 example
Proof of the sequential characterization of limits proof
Decreasing sequence bounded below converges statement
Proof that continuity is equivalent to sequential continuity proof
Limit of a product of sequences statement
Geometric series example
A series of nonnegative terms converges iff its partial sums are bounded statement
Continuity is equivalent to sequential continuity statement
Proof of the root test proof
Sequential characterization of limits statement
Proof of the Cauchy–Hadamard theorem proof
Proof of the ratio test 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
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
Bolzano–Weierstrass theorem statement
Exponentials dominate powers in sequences statement
Chain rule statement
Product rule statement
Linearity of differentiation statement
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
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
Proof of Rolle's theorem proof
Absolutely convergent series converge statement
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
A function with zero derivative is constant statement
Uniform convergence of derivatives allows term-by-term differentiation 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 Pythagorean trigonometric identity by differentiation proof
Pythagorean trigonometric identity statement
Proof of Heron's formula from the law of cosines proof

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Heron's formula

Tag	`PM8PCNPR23`
URI	`m2://PM8PCNPR23`
Kind	proof
Writing	W0 stub
MSC	51M25
Proves	Heron's formula
Uses	Area of a triangle, Triangle, Euclidean distance, Angle at a point, Law of cosines, Sine and cosine, Pythagorean trigonometric identity
Changed	4 November 2026

Proof of Heron's formula from the law of cosines

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