f(x) = 2x + 1
g(x) = x^2 +3
f[g(x)] = 2g(x)+1 = 2(x^2 +3) +1 = 2x^2+7
g[f(x)] = [f(x)]^2+ 3 = (2x+1)^2 +3 = 4x^2 + 4x + 4您中间有没有省略哪一步？没有。。。就是代入就可以了。好吧，，我再琢磨琢磨