设x^2+1 = t那么t>=1
y=4(t-1) + 16/t^2 = 2t + 2t + 16/t^2 -4 >= 3(2t*2t*16/t^2)^1/3 - 4 = 3*8-4=20
当2t=16/t^2也就是t=2的时候取得最小值20
此时x^2 = 1