2/5+1/2×3/5+7/10；12/13×3/7+4/7 用简便方法计算

2/5+1/2×3/5+7/10
=2/5+3/10+7/10
=2/5+1
=7/5
12/13×3/7+4/7
=(9/13+1)×4/7
=22/13×4/7
=88/91
再问： =(9/13+1)×4/7 =22/13×4/7 =88/91 9/13是怎样出来的？
再答： 12/13×3/7+4/7 =3×4/13×3/7+4/7 =3×3/13×4/7+4/7 =(9/13+1)×4/7 =22/13×4/7 =88/91
再问： 还是不明白啊，3×3/13×4/7+4/7怎么做的？
再答： 12/13×3/7+4/7 =3×4/13×3/7+4/7 =3×3/13×4/7+4/7 =9/13×4/7+4/7 提取公因数4/7 =(9/13+1)×4/7 =22/13×4/7 =88/91
再问： =3×3/13×4/7+4/7 中3/13×4/7怎样出来的？我愚笨还是不明白，请详细说明
再答： 12/13×3/7+4/7 =3×4/13×3/7+4/7 =(3×3×4)/(13×7)+4/7 (乘法交换律，分子交换） =(9×4)/(13×7)+4/7 =9/13×4/7+4/7