由于x=2^m+1故2^m=x-1两边平方(2^m)^2=4^m=x^2-2x+1所以y=3+x^2-2x+1=x^2-2x+4
2^m=x-1(2^m)²=（x-1）² 即是4^m=（x-1）²代入y式y=3+(x-1）²
