若g(x)=1-2x,f[g(x)]=(1-x^2)/x^2,则f(1/2)的值为f[g(x) ]=(1-x^2)/x^2 令g(x)=1/2 1-2x=1/2 2x=1/2 x=1/4 f(1/2)=(1-x^2)/x^2 令x=1/4 有:f(1/2)=(1-(1/4)^2)/(1/4)^2 =15