f(2) = f(1+1) = 1^2 + 2*1 + 3 = 6没有过程吗？这不就是过程吗？很详细啊，x=1代入用换元法好吧，令t=x+1，则x=t-1
f(t) = (t-1)^2 + 2(t-1) + 3 = t^2 + 2
f(2) = 2^2 + 2 = 6