Question

EM CẦN GẤP Ạ, SÁNG MAII E ĐI HỌC

D = 1/2 + 1/6 + 1/12 +1/20 +1/30 + 1/42 + 1/56 + 1/72 + 1/90

Answer 1

`D = 1/2 + 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + 1/56 + 1/72 + 1/90`

`= 1/(1*2)+1/(2*3)+1/(3*4)+1/(4*5)+1/(5*6)+1/(6*7)+1/(7*8)+1/(8*9)+1/(9*10)`

`= 1 - 1/2 + 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 - 1/10`

`= 9/10`

Answer 2

``D = 1/2 + 1/6 + 1/12 + ... + 1/90`

D = 1/(1.2) + 1/(2.3) + 1/(3.4) + ... + 1/(9.10)`

`D = 1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + ... + 1/9 - 1/10`

`D = 1 - 1/10`

`D = 9/10`