Question

tính tổng

1 + ( - 2 ) + 3 + ( - 4 ) + ... + 19 + ( - 20 )

Answer 1

Đặt A = 1 + ( - 2 ) + 3 + ( - 4 ) + .... + 19 + ( - 20 )

=> A = [ 1 + ( - 2 ) ] + [ 3 + ( - 4 ) ] + .... + [ 19 + ( - 20 ) ]

=> A = ( - 1 ) + ( - 1 ) + .... + ( - 1 ) [ Có 10 thừa số - 1 ]

=> A = ( - 1 ) . 10

=> A = - 10

Vậy A = - 10

Answer 2

1 + ( - 2 ) + 3 + ( - 4 ) + ... + 19 + ( -20 )

= [ 1 + ( -2 ) ] + [ 3 + ( -4 ) ] + ... + [ 19 + ( -20 ) ]

= ( - 1 ) + ( - 1 ) + ... + ( -1 )