Algebra and Trigonometry 10th Edition

In f(x), all the terms in the polynomial have powers of x as multiples of 2. Hence, f(x)=f(-x), for replacing x with -x in the polynomial will result in exactly the same terms as $x^{2k}=(-x)^{2k}$ for k =2,4,....,2n (k is even). Hence the function is even.