# Thread: Help with a Trig Proof

1. ## Help with a Trig Proof

The question says:

Given y = xsinx, prove that y" + y' = 2cosx.

I found that y' = sinx + xcosx, and y" = 2cosx - xsinx.

But I cannot figure out how to simplify (2cosx - xsinx) + (sinx + xcosx) so that it equals 2cosx.

Help?

Jeannine

2. Hi,
Originally Posted by jbecker007
The question says:

Given y = xsinx, prove that y" + y' = 2cosx.

I found that y' = sinx + xcosx, and y" = 2cosx - xsinx.
That's correct.
But I cannot figure out how to simplify (2cosx - xsinx) + (sinx + xcosx) so that it equals 2cosx.
I think the question should be "Given $y = x\sin x$, prove that $y'' + y = 2\cos x$".