Question

tính A=1/2+1/6+1/12+1/20+1/30+1/42 (nhớ trình bày cách giải nhé)

Answer 1

A=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

A=1-1/7=6/7

Answer 2

=1/1.2 + 1/2.3 + 1/3.4 +1/4.5 +1/5.6 +1/6.7

=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)+(1/2-1/2)+(1/3-1/3) + ... +(1/6-1/6)

= (1-1/7) + 0+...+0 = 1-1/7 = 7/7 - 1/7 = 6/7

Vậy A =6/7