括号7/29×30+7÷29括号×(2/31)=（7/29x30+7/29）x2/31=7/29x(30+1)x2/31=7/29x(31x2/31)=7/29x2=14/29
（7／29x30＋7➗29）x（2／31）＝（30＋1）x7／29x2／31＝14／29
