7/8+5/7+2/7简便计算=5/7+2/7+7/8=1+7/8=1又7/8
7/8+5/7+2/7=7/8+（5/7+2/7）=7/8+1=1又8分之7
7/8十（5/7十2/7）=7/8十1=15/8
7/8+（5/7+2/7）=7/8+（1）=1+7/8=1又8分之7
7/8+5/7+2/7=7/8+(5+2)/7=7/8十1=15/8
