Question

Answer 1

a. 1/2 + 1/6 + 1/12 + 1/20 + 1/30
=1/1x2 + 1/2x3 + 1/3x4 + 1/4x5 + 1/5x6
=1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + 1/5 - 1/6
=1 - 1/6 = 5/6

b. 2/8 + 2/24 + 2/48 + 2/80 + 2/120
= 2/2x4 + 2/4x6 + 2/6x8 + 2/8x10 + 2/10x12
= 1/2 - 1/4 + 1/4 - 1/6 + 1/6 - 1/8 + 1/8 - 1/10 + 1/10 - 1/12
= 1/2 - 1/12 =  6/12 - 1/12 = 5/12

Answer 2

$#Shả$

`a)=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6`

`=1-1/6=5/6`

`b)=2/(2xx4)+2/(4×6)+2/(6×8)+2/(8×10)+2/(10×12)`

`=1-2/4+2/4-2/6+2/6-2/8+2/8-2/10+2/10-2/12`

`=1-2/12=1-1/6=5/6`