1/a+2/b+3/c
=(1/a)+(1/b)+(1/b)+(1/c)+(1/c)+(1/c)
>=6/(abbccc)^(1/6)
>=6/[(1/6)(a+b+b+c+c+c)]
=36/(a+2b+3c)
36/36
=1
最小值=1