nhanh lên ạ 113 !!!!!!

`E=1/6+2/15+3/40+4/96+5/204+6/391+7/690`

`= 1/(2×3)+2/(3×5)+3/(5×8)+4/(8×12)+5/(12×17)+6/(17×23)+7/(23×30)`

`= 1/2-1/3+1/3-1/5+1/5-1/8+1/8-1/12+1/12-1/17+1/17-1/23+1/23-1/30`

`= 1/2-1/30`

`= 15/30-1/30`

`= 14/30 = 7/15`