指数函数则f(x)=a^x
f(-2/3)=a^(-2/3)=4^(1/3)
a^(-2/3)=2^(2/3)
a^(-2/3)=(1/2)^(-2/3)
a=1/2
f(-1/2)=(1/2)^(-1/2)
=2^(1/2)
=根号2