How do you test the improper integral int sintheta/cos^2thetasinθcos2θ from [0,pi/2][0,π2] and evaluate if possible?

1 Answer
Aug 14, 2017

Use limits.

Explanation:

The integral is improper where costheta = 0cosθ=0, which occurs at theta = pi/2θ=π2.
To evaluate,
int_0^(pi/2) sintheta/cos^2theta d(theta)π20sinθcos2θd(θ)
first observe that sintheta/cos^2theta = sec(theta)tan(theta)sinθcos2θ=sec(θ)tan(θ)
Use a limit on the improper end:
int_0^(pi/2) sec(theta)tan(theta) d(theta) = lim(int_0^bsec(theta)tan(theta) d(theta) ), where the limit is as b -> (pi/2)^-
Evaluate:
lim(int_0^b sec(theta)tan(theta) d(theta) ) =
= lim(secb - sec0)
= lim(secb - 1)
This limit diverges as b -> pi/2.