# trigonometric identity related to $\sum_{n=1}^{\infty}\frac{\sin(n)\sin(n^{2})}{\sqrt{n}}$

As homework I was given the following series to check for convergence:

$\displaystyle \sum_{n=1}^{\infty}\dfrac{\sin(n)\sin(n^{2})}{\sqrt{n}}$

and the tip was "use the appropriate identity".

I'm trying to use Dirichlet's test and show that it's the product of a null monotonic sequence and a bounded series, but I can't figure out which trig. identity is needed.

Can anyone point me towards the right direction?

Many thanks.

Hint: You can show that $$\sum\limits_{n=1}^N\sin(n)\sin(n^2)=\frac{1}{2}(1-\cos(N^2+N))$$ To do this use identity $$\sin(\alpha)\sin(\beta)=\frac{1}{2}(\cos(\alpha-\beta)-\cos(\alpha+\beta))$$