$$ \int \sin(x) \cos(x)\; dx $$
If i take $u = \sin(x)$ I get final answer to be $\sin^2(x) / 2 + c$
But If i take $u = \cos(x)$ I get final answer to be $-\cos^2(x) / 2 + c$
Are they equal? They should be, otherwise it does not make sense. But how are they equal?