F(2x+1)=x^2+2x-1 ;
x=2;f(5) =2²+2×2-1=7;
F(2x+1)=x^2-2x+3
=(x-1)²+2
=((2x+1)/2-3/2)²+2;
则f（x）=(x/2-3/2)²+2=x²/4-3x/2+17/4;
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