f(x-1)=x^2，注意自变量是x-1，函数表达式中只能出现x-1形式f(x-1)=x^2=(x-1+1)²=(x-1)²+2(x-1)+1即f(x)=x^2+2x+1
令t=x-1 则f(x-1)=f(t) x=t+1 x^2=(t+1)^2 所以f(t)=(t+1)^2 即f(x)=(x+1)^2
f(x-1)=x²=x²-2x+1+2x-1=(x-1)²+2x-2+1=(x-1)²+2(x-1)+1
