Question

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

Answer 1

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

追问

=(9/13+1)×4/7
=22/13×4/7
=88/91
9/13是怎样出来的？

追答

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

追问

还是不明白啊，3×3/13×4/7+4/7怎么做的？

追答

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

追问

=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

追答

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

Answer 2

（3/10+7/10）+2/5=7/5
(4/7)*9/13+4/7=4/7*(1+9/13)=4/7*21/13=12/13

Answer 3

成他们分母的最小公倍数