求几道题的简便算法..（6.4×7.5×8.1）÷（3.2×25×2.7）1/2+1/6+1/12+1/20+1/30+1/42+1/56把解法写下来袄...顺便把解题过程也写下来.

Question

求几道题的简便算法..
（6.4×7.5×8.1）÷（3.2×25×2.7）
1/2+1/6+1/12+1/20+1/30+1/42+1/56
把解法写下来袄...顺便把解题过程也写下来.

Answer 1

（6.4*7.5*8.1）/（3.2*25*2.7）
=(6.4/3.2)*(7.5/25)*(8.1/2.7)
=2*0.3*3
=1.8
1/2+1/6+1/12+1/20+1/30+1/42+1/56
=1/1*2+1/2*3+1/3*4+1/4*5+1/5*6+1/6*7+1/7*8
=(1/1-1/2)+(1/2-1/3)+……+)1/6-1/7)+(1/7-1/8)
=1/1-1/8
=7/8