\text{By writing }(cosx)^{2n}=(1-sin^2x)^n\text{, show that }\int_{0}^{\frac{\pi}{2}}(cosx)^{2n+1}dx=\sum_{k=0}^{n}\frac{(-1)^k}{2k+1}\binom{n}{k}
I can see why it would equal that, but I just can't get there >.< Probably something obvious but I'm just not seeing it.