13时20分减12时30分等多少 1/12+1/20+1/30+1/42+/56+1/72+1/90（用简便方法计算，要有详细过程）

1/12+1/20+1/30+1/42+/56+1/72+1/90（用简便方法计算，要有详细过程）  

1/12+1/20+1/30+1/42+/56+1/72+1/90（用简便方法计算，要有详细过程）

1/12+1/20+1/30+1/42+1/56+1/72+1/90
=(1/3-1/4)+(1/4-1/5)+(1/5-1/6)+(1/6-1/7)+(1/7-1/8)+(1/8-1/9)+(1/9-1/10)
=1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10
=1/3-1/10
=7/30

1/12+1/20+1/30+1/42+1/56+1/72+1/9简便计算

答：1/12+1/20+1/30+1/42+1/56+1/72+1/9
=（1/3-1/4）+（1/4-1/5）+（1/5-1/6）+（1/6-1/7）+（1/7-1/8）+（1/8-1/9）+1/9
=1/3-1/9+1/9
=1/3

1/20+1/30+1/42+1/56+1/72+1/90的简便方法

原式=1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9+1/10
=1/4-1/10
=3/20

简便运算：1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90

1/(2x3)+1/(3x4)+1/(4x5)+1/(5x6)+1/(6x7)+1/(7x8)+1/(8x9)+1/(9x10)
=1/2-1/3+1/3-1/4.......+1/9-1/10
=1/2-1/10=0.4

1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90 （简便运算）

1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90
=(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+(1/5-1/6)+(1/6-1/7)+(1/7-1/8)+(1/8-1/9)+(1/9-1/10)
=1/2-1/10
=4/10
=2/5

1/12+1/20+1/30+1/42+1/56+1/72简便计算

用裂项法
1/12+1/20+1/30+1/42+1/56+1/72
=1/3-1/4+1/4-1/5+...+1/8-1/9
=1/3-1/9
=2/9

1/12+1/20+1/30+1/42+1/56+1/72+1/90

=1/3*4+1/4*5+1/5*6+1/6*7+1/7*8+1/8*9+1/9*10
=1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10
=1/3-1/10
=7/30
【通项：
1/n(n+1)=1/n-1/n+1】