Math 2.0

statement proposition

Integral of 1/x

Statement

Let a,ba, b be real numbers of the same sign, both nonzero. Then ∫abdxx=ln⁡ba.\int_a^b \frac{dx}{x} = \ln \frac{b}{a}. In particular, ∫1xdtt=ln⁡x\int_1^x \frac{dt}{t} = \ln x for x>0x > 0.

Remarks

The function ln⁡∣x∣\ln |x| is an antiderivative of 1/x1/x on each of (0,∞)(0, \infty) and (−∞,0)(-\infty, 0), by the derivative of the logarithm. Some texts use the second formula as the definition of the natural logarithm. The integral over an interval that contains 00 does not exist. Mathlib states it as integral_inv, under the hypothesis 0∉[a,b]0 \notin [a, b].

Explanations

L1 human

The power rule fails for x−1x^{-1}, since it would divide by 00. The logarithm fills this gap: its derivative is 1/x1/x.

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To read this node, read these 64 nodes in this order. The foundations come first, and the list ends with this node.

  1. Limit point of a set definition
  2. Local maximum definition
  3. Partition of an interval definition
  4. Sequence definition
  5. Strictly monotone function definition
  6. Limit of a function definition
  7. Local minimum definition
  8. Mesh of a partition definition
  9. Riemann sum definition
  10. Real sequence definition
  11. Subsequence definition
  12. Continuity at a point definition
  13. Derivative definition
  14. One-sided limit definition
  15. Riemann integrable function definition
  16. Bounded sequence definition
  17. Geometric sequence definition
  18. Limit of a sequence definition
  19. Monotone sequence definition
  20. Series definition
  21. Continuous function definition
  22. Epsilon-delta definition of continuity statement
  23. Fermat's theorem on stationary points statement
  24. Sum of a finite geometric series statement
  25. Monotone convergence theorem for sequences statement
  26. Convergent sequence definition
  27. Every sequence has a monotone subsequence statement
  28. Squeeze theorem statement
  29. Cauchy product definition
  30. Partial sum definition
  31. Power series definition
  32. Intermediate value theorem statement
  33. Composition of continuous functions is continuous statement
  34. One-sided limits of a monotone function exist statement
  35. Bolzano–Weierstrass theorem statement
  36. The sequence r^n tends to zero for |r| < 1 example
  37. Radius of convergence definition
  38. Convergent series definition
  39. Chain rule statement
  40. Boundedness theorem statement
  41. Continuity of the inverse of a strictly monotone continuous function statement
  42. A series of nonnegative terms converges iff its partial sums are bounded statement
  43. Absolute convergence definition
  44. Sum of a series definition
  45. Extreme value theorem statement
  46. Derivative of an inverse function statement
  47. Comparison test statement
  48. Geometric series example
  49. Cauchy product of absolutely convergent series statement
  50. Rolle's theorem statement
  51. Ratio test statement
  52. Mean value theorem statement
  53. Ratio formula for the radius of convergence statement
  54. A function with positive derivative is strictly increasing statement
  55. Second fundamental theorem of calculus statement
  56. The exponential series has infinite radius of convergence statement
  57. Exponential function definition
  58. Exponential of a sum statement
  59. The exponential function is positive statement
  60. The exponential function is its own derivative statement
  61. The exponential function is strictly increasing statement
  62. Natural logarithm definition
  63. Derivative of the logarithm statement
  64. Integral of 1/x statement

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Sources

  1. Web https://en.wikipedia.org/wiki/Natural_logarithm, Integration