a=2^m+1
2^m=a-1
y=3+4^m
=3+(2×2)^m
=3+(2^m)×(2^m)
=3+(a-1)×(a-1)
=3+a^2-2a+1
=a^2-2a+4
前面的a是x吧.
=x^2-2x+4