令t=x-1，则x=t+1f(t)=(t+1)^2+2(t+1)-3=t^2+4tt和x同样在定义域上取值。f(x)=x^2+4x
f(x-1)=x^2-2x+1+4x-4=(x-1)^2+4(x-1)f( x ) = x ^2+4 x
令x-1=t,带入原式中得f(t)=(t+1)^2+2(t+1)-3,整理得f(x)=x^2+4x
