设f(x)对任意的x,y都有f(x+y)+f(x-y)=2f(x)+f(y)且f(0)≠0,求证f(x)是偶函数

问题描述:

设f(x)对任意的x,y都有f(x+y)+f(x-y)=2f(x)+f(y)且f(0)≠0,求证f(x)是偶函数

f(y) = f(x+y)+f(x-y) -2f(x)
f(-y) = f(x-y)+f(x+y) -2f(x) =f(y)
f(x)是偶函数