1/(1/15+1/20)=1÷(4/60+3/60)=1÷7/60=60/7
先求分母1/15+1/20 =4/60 +3/60=7/60再求倒数 => 60/7
1/(1/15+1/20)=60/7
原式=1/[(4+3)/60] =60/7希望能帮到你❀\(^o^)/~
60/7