Tính nhanh: 1/1x2 + 1/2x3 + 1/3x4 + 1/4x5 + 1/5x6 + 1/6x7

Tính nhanh: a) 1/1x2 + 1/2x3 + 1/3x4 + 1/4x5 + 1/5x6 + 1/6x7 b) 1/2 + 1/4 + 1/8 + 1/16 + 1/32 + 1/64