因为3a+b=2,所以3a/2+b/2=1,则1/a+1/3b=(1/a+1/3b)(3a/2+b/2)=3/2+b/2a+a/2b+1/6=5/3+b/2a+a/2b>=5/3+2根号下(b/2a*a/2b)=5/3+1=8/3.故最小值为8/3,采用均值不等式.