Tính A= cos10 + cos20 + cos30 +.....+cos180 B= tan20 + tan40 + tan 60 +....+ tan160 + tan180

`A = cos 10^@ + cos 20^@ + cos 30^@ + .... + cos 180^@`

`= cos 10^@ + cos 20^@ + .... + cos 80^@ + cos 90^@ - cos 80^@ - cos 70^@ - .... - cos10^@ + cos 180^@`

`= (cos 10^@ - cos 10^@) + (cos 20^@ - cos 20^@) + ... + (cos 80^@ - cos 80^@) + cos 90^@ + cos 180^@`

`= cos 90^@ + cos 180^@`

`= 0 - 1`

`= -1`

Vậy `A =- 1`

`B = tan 20^@ + tan 40^@ + tan 60^@ + ... + tan 160^@ + tan 180^@`

`= tan 20^@ + tan 40^@ + .... + tan 80^@ - tan 80^@ - tan 60^@ - ... - tan 20^@ + tan 180^@`

`= (tan 20^@ - tan 20^@) + (tan 40^@ - tan 40^@) + ... + (tan 80^@ - tan 80^@) + tan 180^@`

`= tan 180^@`

`= 0`

Vậy `B = 0`