4^x+2^(x+1)=80
2^2x+2*2^x=80
(2^x)^2+2*2^x=80
设2^x=y,则
y^2+2y=80
y^2+2y+1=81
(y+1)^2=81
y+1=±9
y= -1±9
y=8 或 -10
2^x=8 ,x=3
或2^x = -10 舍去