TínhA=1/3+1/6+1/10+....+1/190

A:2 = 1/6 + 1/12 + 1/20 +...+ 1/380 = 1/(2x3) + 1/(3x4) + 1/(4x5) + ... + 1/(19x20) = 1/2 - 1/3+1/3 - 1/4 +....+ 1/19- 1/20 = 1/2 - 1/20 = 9/20 Suy ra A = 9/20 x 2 = 9/10