MHB Can You Derive a Reduction Formula for the Integral $\int \sec^nx dx$?

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SUMMARY

The discussion focuses on deriving a reduction formula for the integral $\int \sec^n x \, dx$ for values of $n \ge 2$. The suggested solution involves integration by parts, where the integral is expressed in terms of $\sec^{n-2} x$ and a trigonometric identity is utilized. The final reduction formula is established as $\int \sec^n x \, dx = \frac{1}{n-1} \sec^{n-2} x \tan x + \frac{n-2}{n-1} \int \sec^{n-2} x \, dx$.

PREREQUISITES
  • Understanding of integration techniques, specifically integration by parts.
  • Familiarity with trigonometric identities, particularly those involving secant and tangent functions.
  • Knowledge of the properties of definite and indefinite integrals.
  • Basic algebraic manipulation skills for handling expressions involving powers of secant.
NEXT STEPS
  • Study the derivation of integration by parts in detail.
  • Explore trigonometric identities relevant to integrals involving secant and tangent.
  • Practice deriving reduction formulas for other trigonometric integrals.
  • Investigate applications of reduction formulas in solving complex integrals.
USEFUL FOR

Students and educators in calculus, mathematicians focusing on integral calculus, and anyone interested in advanced techniques for solving integrals involving trigonometric functions.

lfdahl
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Derive a reduction formula for the integral:

$\int \sec^nx dx, \;\;\; n \ge 2.$

- without any help from an online integrator.
 
Last edited:
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Suggested solution:

\[\int \sec^nxdx \\\\= \int \sec^2x\sec^{n-2}xdx = \tan x \sec^{n-2}x-(n-2)\int \tan x \sec^{n-3}x \sec^{2}x \sin xdx \\\\= \tan x \sec^{n-2}x-(n-2)\int \sin^2 x\sec^nxdx \\\\= \tan x \sec^{n-2}x-(n-2)\int (1-\cos^2x)\sec^nxdx \\\\= \tan x \sec^{n-2}x-(n-2)\int \sec^nxdx+(n-2)\int \sec^{n-2}xdx \\\\ \Rightarrow \int \sec^nxdx = \frac{\tan x \sec^{n-2}x}{n-1}+\frac{(n-2)}{(n-1)}\int \sec^{n-2}xdx\]
 

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