I need to represent $\cos(x^{1/2})$ by Maclaurin series.
I'm not sure that what I have done is correct. We know Maclaurin series for $\cos(x) = 1- {x^2 \over 2!} +{x^4\over 4!}+\cdots$ So I substitue $x^{1/2}$, and I got $\cos(x^{1/2})= 1 - {x\over 2!} + {x^2\over4!}+\cdots$
But when I check it, with $x=16$, I don't get the correct answer. Where is the problem? Thanks.