Problem: [int]cosx(sinx)dx Given: x=pi; f(pi)=13.4 I am utterly confused on how to solve this integral. I am 99% positive (which is nothing in the math world) that I need to apply the product rule to all of this in order to find the antiderivative. However, no matter how I think of going about it, I will always have that addition sign in there that ruins the whole equation so I can't divide/null particular variables in order to end up with the cosx(sinx) as the derivative. What I have done so far is firstly finding the antiderivatives for cosx and sinx. I know cosx=sinx + C and sinx=-cosx + C. But now I do not know the next step. I have solved other integral problems like e^2x and the like, so it's not a foreign concept, but with this one I have no clue. I have thought about using the sum and difference identities for cos and sin: sin2x=sinx(cosx) + sinx(cosx) cos2x=(cosx)^2 - (sinx)^2 Although once again, I couldn't figure out how those could help me, in addition to applying the quotient rule... however, like I said previously, I am almost positive I need to be using the product rule to solve this integral function, but then what do I do? I know some of you have suggestions, please help, this is driving me mad!