Question

Tính hợp lí:

a ) A = 1 6   + 1 12   + 1 20   + 1 30   + 1 42   + 1 56  

b ) B = 3 4   + 3 28   + 3 70   + 3 130   + 3 208   + 3 304

c ) C = 1 4   + 1 28   + 1 70   + 1 130   + 1 208   + 1 304

d ) D = 1 2   + 1 14   + 1 35   + 1 65   + 1 104   + 1 152

Answer 1

a ) A = 1 2.3 + 2 3.4 + 1 4.5 + 1 5.6 + 1 6.7 + 1 7.8

1 2 − 1 3 + 1 3 − 1 4 + 1 4 − 1 5 + 1 5 − 1 6 + 1 6 − 1 7 + 1 7 − 1 8 = 3 8

b ) B = 3 1.4 + 3 4.7 + 3 7.10 + 3 10.13 + 3 13.16 + 3 16.19

= 1 − 1 4 + 1 4 − 1 7 + 1 7 − 1 10 + 1 10 − 1 13 + 1 13 − 1 16 + 1 16 − 1 19 = 18 19

c ) C = 1 3 3 1.4 + 3 4.7 + 3 7.10 + 3 10.13 + 3 13.16 + 3 16.19

= 1 3 1 − 1 4 + 1 4 − 1 7 + 1 7 − 1 10 + 1 10 − 1 13 + 1 13 − 1 16 + 1 16 − 1 19 = 6 19

d ) D = 2 4 + 2 28 + 2 70 + 2 130 + 2 208 + 2 304

= 2 3 3 1.4 + 3 4.7 + 3 7.10 + 3 10.13 + 3 13.16 + 3 16.19 = 12 19