i'm confused on which item should be my u and which should be my du, i've tried a few values on all problems but I feel my answer is wrong and that I'm overlooking something, if you could please help I would be eternally grateful!

Use U substitution to solve the following:

a)anti derivative of cos(x)/sin^2(x) dx top valuei/2 bottom value: pi/4

b) (4(tan^-1(x))^2)/(1+x^2) dx top value: 1 bottom value: -1

c) 5x(3x^2 - 12)^9 dx top value: 4 bottom value: 0