(5x)Σcos(x) cosu^4 du=
I got 5cos(20x^3)+cos(4cos(x)^3)(sin(x)) but it is incorrect. can someone explain to me why this is incorrect help me correct it?
With that "du" I'm inclined to believe you mean an integration with respect to u But then what about the "5x" and "cos(x)"? Do you mean, the simplest way I can interpret this,
$\displaystyle 5x cos(x) \int cos^4(u) du$? To integrate that you could use $\displaystyle cos^2(u)= (1/2)(1+ cos(2u))$, twice. But I cannot imagine where you could have got that "20" inside the cosine, much less the cube! So perhaps I am misunderstanding completely.