Math 2.0

Real functions Proof

Proof of the constant multiple rule

Builds on Linearity of differentiation

Proof

We deduce the rule from the linearity of differentiation.

  1. Apply linearity to the functions ff and g=fg = f, which are both differentiable at aa, with α=c\alpha = c and β=0\beta = 0.
  2. For each x∈Ix \in I, (cf+0⋅f)(x)=cf(x)(c f + 0 \cdot f)(x) = c f(x). So cf+0⋅f=cfc f + 0 \cdot f = c f.
  3. Linearity gives that cfc f is differentiable at aa, with (cf)′(a)=cf′(a)+0⋅f′(a)=cf′(a)(c f)'(a) = c f'(a) + 0 \cdot f'(a) = c f'(a).

Idea

A constant multiple is the special case of a linear combination where the second coefficient is zero. So the rule is a direct case of linearity.

Reading path

To understand this page from the ground up, read these 68 pages in order. The foundations come first. The path takes the proofs Proof of the limit of a sum of sequences, Proof of the approximation property of the supremum, Proof that there is a natural number n with 1/n below any positive epsilon, Proof that the reals form an ordered field, with the Cauchy sequence construction, Proof of the limit of a product of sequences, Proof that the absolute value is multiplicative, Proof of the linearity of differentiation, Proof of the sequential characterization of limits, Proof of the triangle inequality for real numbers, Proof of the epsilon-N definition of the limit of a sequence, Proof of the epsilon-delta form of the limit of a function, Proof that the reals are Archimedean from the completeness axiom, Proof that convergent sequences are bounded.

  1. Construction of the reals from Cauchy sequences concept
  2. Element definition
  3. Integers object
  4. Limit point of a set definition
  5. Ordered field definition
  6. Proposition concept
  7. Rational numbers object
  8. Real numbers object
  9. Sequence definition
  10. Set bounded above definition
  11. Set definition
  12. Ring of integers statement
  13. Limit of a function definition
  14. Archimedean ordered field definition
  15. Implication definition
  16. Predicate definition
  17. The reals form a field statement
  18. Real sequence definition
  19. Supremum definition
  20. Ordered pair definition
  21. Derivative definition
  22. The rationals form a field statement
  23. Universal quantifier definition
  24. Bounded sequence definition
  25. Limit of a sequence definition
  26. Completeness axiom statement
  27. Cartesian product definition
  28. Differentiable function definition
  29. Subset definition
  30. The rationals form an ordered field statement
  31. Convergent sequence definition
  32. Binary relation definition
  33. Order on the reals definition
  34. Antisymmetric relation definition
  35. Reflexive relation definition
  36. Transitive relation definition
  37. Interval definition
  38. Preorder definition
  39. Proof of the epsilon-delta form of the limit of a function proof
  40. Partial order definition
  41. Epsilon-delta definition of a limit statement
  42. Total order definition
  43. Proof that the reals form an ordered field, with the Cauchy sequence construction proof
  44. The reals form an ordered field statement
  45. Absolute value definition
  46. Proof of the approximation property of the supremum proof
  47. Proof of the triangle inequality for real numbers proof
  48. Proof that the absolute value is multiplicative proof
  49. Proof of the epsilon-N definition of the limit of a sequence proof
  50. Approximation property of the supremum statement
  51. Triangle inequality statement
  52. Absolute value is multiplicative statement
  53. Epsilon-N definition of the limit of a sequence statement
  54. Proof that the reals are Archimedean from the completeness axiom proof
  55. Proof of the limit of a sum of sequences proof
  56. Proof that convergent sequences are bounded proof
  57. The reals are Archimedean statement
  58. Proof that there is a natural number n with 1/n below any positive epsilon proof
  59. Limit of a sum of sequences statement
  60. Convergent sequences are bounded statement
  61. There is a natural number n with 1/n below any positive epsilon statement
  62. Proof of the limit of a product of sequences proof
  63. Proof of the sequential characterization of limits proof
  64. Limit of a product of sequences statement
  65. Sequential characterization of limits statement
  66. Proof of the linearity of differentiation proof
  67. Linearity of differentiation statement
  68. Proof of the constant multiple rule proof

Where this leads

1083 pages build on this one.

Further reading

  1. Web https://en.wikipedia.org/wiki/Linearity_of_differentiation, Constant coefficient
About this page
TagZW8Z49W7NT
URIm2://ZW8Z49W7NT
Kindproof
WritingW0 stub
MSC26A24
ProvesConstant multiple rule
UsesLinearity of differentiation
Changed4 November 2026

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