A= 1/2 + 1/6 +1/12+1/20+1/30+...+1/n A=49/50 Tìm n...

- Hướng dẫn giải

1/2 = 1/(1x2) = 1 - 1/2

1/6 = 1/(2x3) = 1/2 - 1/3

1/12 = 1/(3x4) = 1/3 - 1/4 ........ 1/n = 1/(nx(n+1)) = 1/n - 1/(n+1) 1 /2 + 1/6 + 1/12 + 1/20 + 1/30 +...+ 1/n = 1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 +...+1/n - 1/(n+1) = 49/50 Hay A = 1 - 1/(n+1) = 49/50 => 1/(n+1) = 1 - 49/50 1/(n+1) = 1/50 Suy ra n+1=50 nên n=49