Trig subtitution

by gigi9
Tags: subtitution, trig
gigi9 is offline
Nov8-03, 09:18 AM
P: 43
Can someone plz show me the concept of trig substituion and easy way to use it...I don't quite understand this...
I'm ot sure how to dothis problem below..
1) indefinite integral dx/(x^3sqrt(x^2-a^2))
2) indefinite integral (1-4x^2)^(3/2)dx

How to use integration by part for
indefinite integral of x*inverse tan(x) dx.
Phys.Org News Partner Science news on
Internet co-creator Cerf debunks 'myth' that US runs it
Astronomical forensics uncover planetary disks in Hubble archive
Solar-powered two-seat Sunseeker airplane has progress report
PrudensOptimus is offline
Nov8-03, 09:45 AM
P: 640
#1: It is the ∫ Sec-1x * x-2 dx.

Use parts formula: let u = Sec-1, du = x'/|x|sqrt(x^2 - a^2) where a = 1. dv = x-2 dx, v = -x^-1.

The answer? I'm not sure, but out of scratch i got -sec^-1x/x + 2sqrt(x^6 - x^4)/6x^5 - 4x^3 + C = -Cosx/x - sinx * x^3/3
HallsofIvy is offline
Nov9-03, 07:00 AM
Sci Advisor
PF Gold
P: 38,904
The point of "trigonometric substitution" is to use trig identities such as
sin2(θ)= 1- cos2(θ) so that
sin(θ)= √(1- cos2(θ))

sec2(θ)= 1+ tan2(θ) so that
sec(θ)= √(1+ tan2(θ))

tan2(θ)= sec2(θ)- 1 so that
tan(θ)= √(sec2(θ)- 1)

When you see something like ∫dx/(x3√(x2-a2)) you should immediately think "Hmmm, that √(x2- a2) reminds me of √(sec2(θ)- 1)- especially if I factor out a2 to get
a√((x/a)2-1). I'll bet making a substitution like
x/a= sec(θ) (0r x= a sec(θ)) will work!

Doing that, √(x2- a2) becomes √(a2sec2(θ)- a2)= a√(sec2-1)= a√(tan2)= a tan(θ)!

Also, x3 become a3sec3(θ) and dx= a d(sec(θ))= a sec(θ)tan(θ) dθ

The entire integral becomes ∫(a sec(θ)tan(θ)dθ/(a3sec3(θ)(a tan(θ)))
= (1/a3)∫(1/sec2(θ)
= (1/a3)∫cos2(θ)dθ
which can be done by using the trig identity:
cos2(θ)= (1/2)(1+ cos(2θ)).

phoenixthoth is offline
Nov9-03, 02:18 PM
phoenixthoth's Avatar
P: 1,572

Trig subtitution

i was thinking about that and actually shouldn't it be |atanθ|, not atanθ? the a vs |a| part doesn't matter because with the x3, you'd have a3|a|=a4 anyway but there'd still be a |tanθ|. perhaps when the integration is done and you go back to x, it won't matter if it was tanθ or |tanθ|...

Register to reply

Related Discussions
more trig.... Precalculus Mathematics Homework 3
another trig sub Calculus & Beyond Homework 3
trig trig trig Introductory Physics Homework 11
trig sub. Introductory Physics Homework 5
trig Introductory Physics Homework 1