先令x1 = x2 = 0可得f(0) = -1;
f(x - x) = f(x) + f(-x) + 2x^2 + 1 ==> f(0) = f(x) + f(x) + 2x^2 + 1
f(x + x) = f(x) + f(x) -2x^2 + 1
两式相减得：
f(0) - f(2x) = 4x^2 = (2x)^2
即：
f(2x) = f(0) - (2x)^2
所以f(x) = -1 - x^2
f(x)出来了那其他的就好办了,求导可知F(x)有最小值3