Let
I=∫cosec3 x dx =∫cosec xI⋅cosec2 xII dx
Applying by parts, we get
I=cosec x(−cotx)−∫(−cosec xcotx)(−cotx) dx=−cosec xcotx−∫cosec x(cosec2 x−1) dx=−cosec xcotx−∫cosec3 x dx+∫cosec x dx⇒I=−cosec xcotx−I+ln|cosec x−cotx|+c∴I=−cosec xcotx+ln|cosec x−cotx|2+C