Question

tính nhanh : B = 1/2 + 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + .... + 1/110

Answer 1

B = 1/2 + 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + ... + 1/110

B = 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 + 1/5.6 + 1/6.7 + .... + 1/10.110

B = 1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + 1/5 - 1/6 + 1/6 - 1/7 + ... + 1/10 - 1/11

B = 1 - 1/11

B = 10/11

Answer 2

 B = 1/2 + 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + .... + 1/110

=1/1x2 + 1/2x3 + 1/ 3x4 + 1/4x5 + 1/5 x6 + 1/ 6x 7 +.....+ 1/10 x11

=1-1/2 + 1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7 +......+ 1/10 - 1/11

= 1 + (1/2-1/2)+(1/3-1/3)+(1/4-1/4)+(1/5-1/5)+(1/6-1/6)+.....+(1/10-1/10)-1/11

= 1- 1/11=10/11

Vậy B = 1/2 + 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + .... + 1/110 = 10/11

Answer 3

cho mình hoi : 1/2 +1/6 +1/12 +1/30 +.....+1/110

cảm ơn nhé

Answer 4

1/2+1/6+1/12+1/20+1/30+1/42......1/110

1/1*2+1/2*3+1/3*4+1/4*5+1/5*6+1/6*7+.....1/11*10

2-1/1*2+3-2/2*3+4-3/3*4+5-4/4*5+6-5/5*6+7-6/6*7.......11-10/10*11

2/1*2-1/1*2+3/2*3-2/2*3+4/3*4-3/3*4+5/4*5-4/4*5+7/6*7-6/67......11/10*11-10/10*11

1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7.......1/11-1/10

1-1/10

10/11