f(x)=x^2-(m+2)x+2m
偶函数不含奇数次项
则必有m+2=0
m=-2
或者根据定义来做：
f(x)=x^2-(2+m)x+2m
f(-x)=x^2+(2+m)x+2m
偶函数f(x)=f(-x)
则
x^2-(2+m)x+2m=x^2+(2+m)x+2m
则2(2+m)x=0
因为要对一切x都成立
则必有2+m=0
m=-2