脱式计算5/8x1/7+6/7x5/9=5/56+10/21=15/168+70/168=85/168
脱式计算，5/8 * 1/7 + 6/7 *5/9=5/56 + 30/63=5/56+10/21分母提取公因式7*8*3=15/56*3 + 80/21*8=95/168
