换元法
令t=√x+1
√x=t-1,t>=1
x=(t-1)^2
f(t)=(t-1)^2+2(t-1)
=t^2-1
f(x)=x^2-1,x>=1
f(x+1)=(x+1)^2-1
=x^2+2x,x>=0,
f(x^2)=((x^2))^2-1=x^4-1,|x|>=1