I need to prove that when .

I'll use Cauchy's version for Lagrange's lemma:

If f,g are continuous in [a,b] and derivative in (a,b), then , (there exists a 'c' : a<c<b for which...)

So I'll take the range [0,x], while , and f(x):=sin(x), g(x):=xcos(x) :

Now, how can I prove that the last phrase is larger than 1? (so that ) ?

Thank you very much!