Question

tinh nhanh 1/2 +1/6 +1/12 +1/20 +1/30 +1/42

Answer 1

=1/1x2+1/2x3+1/3x4+1/4x5+1/5x6+1/6x7

=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7

=1-1/7=6/7