Question

1/2+5/6+11/12+19/20+......+89/90

Answer 1

1/2+5/6+11/12+19/20+...+89/90

                    Đ/s: 81/10.

Answer 2

Đặt A = 1/2 + 5/6 + 11/12 + 19/20 + ... + 89/90

A = ( 1 - 1/2 ) + ( 1 - 1/12 ) + ( 1 - 1/20 ) + ... + ( 1 - 1/90 )

A = ( 1 + 1 + 1 + ... + 1 + 1 ) - ( 1/2 + 1/6 + 1/20 + ... + 1/90

A = 9 - ( 1/1.2 + 1/2.3 + 1/3.4 + ... + 1/9.10 )

A = 9 - ( 1 - 1/10 )

A = 9 - 9/10

A = 81/10