Let the even function be f(x)=cos‌x On differentiating w.r.t. x, we get f′(x)=−sin‌x Again differentiating w.r.t. x, we get f′′(x)=−cos‌x at x=π f′′(π)=−cos‌π=1 ∴ Our assumption is true. at x=−π f′′(−π)=−cos(−π)=1