1.f(√x+1）=x+2√x
f(√x+1）=(√x+1)^2-1
f(x)=x^2-1
2.设f(x）=ax^2+bx+3
f(x+2)-f(x)=4a+4ax+2b=4x+2
a=1 b=-1
f(x）=x^2-x+3