differentiate both and you will see the relationship. of course, it is the same relationship between the derivatives of sine and cosine, since their power series converge. it is just a matter of using the power rule. think of everything but x as a constant, leave the summation sign where it is and differentiate the expression after it

P.S. how is it you can type LaTeX code there, but cannot do it here? all you need to do is put [math[ tags around the code