原式=(9+2/7+7+2/9)÷(5/7+5/9)=(63/7+2/7+63/9+2/9)÷(5/7+5/9)=(65/7+65/9)÷(5/7+5/9)=[65(1/7+1/9)]÷[5(1/7+1/9)]=65÷5=13