3^x+27^y
=3^x+(3^3)^y
=3^x+3^(3y)
=3^x+3^[3*(-1/3x+2/3)]
=3^x+3^(-x+2)
=3^x+9/3^x
≥2√3^x*9/3^x
=2√9
=2*3
=6
当且仅当3^x=9/3^x
3^2x=9
x=1
取到等号
最小值为6