3^a+27^b
=3^a+3^(3b)
≥2√[3^a*3^(3b)]
=2√[3^(a+3b)]
≥2√3^(2√(a*3b))
=2√3^(2*1)
=2√3^2
=2*3
=6
你把3^a*3^(3b)乘错了