1的代换.
1/a+3/b=1
所以a+2b
=(1/a+3/b)(a+2b)
=1+2b/a+3a/b+6
=7+(2b/a+3a/b)
2b/a>0,3a/b>0
所以2b/a+3a/b+7≥2√(2b/a*3a/b)+7=2√6+7
所以最小值=2√6+7
希望能帮助到你.