I believe, many of us had this problem: After finding the integral of some function, we wonder, what the right answer would be. So we goto http://integrals.wolfram.com to look up the answer. The answer the "Integral machine" gave is not always in the same form our answers are. For example: ∫ Cos[x]^3 = u - u^3/3, where u = sin x. The answer the integral machine outputs would be something more nastier... Can someone explain, how do you generally determine whether an expression is identical to another expression(usually in a more complex form)?