Đáp án + Giải thích các bước giải:

Bài 1:

`a)`

`A=cos55^{o}+cos125^{o}=cos55^{o}+cos(\pi-55^{o})=cos55^{o}-cos55^{o}=0`

`b)`

`B=sin30^{o}+cos120^{o}`

`=sin((\pi)/(2)-60^{o})+cos(\pi-60^{o})`

`=cos60^{o}-cos60^{o}`

`=0`

`c)`

`C=sin45^{o}+cos30^{o}-sin135^{o}+cos150^{o}`

`=(sin45^{o}-sin135^{o})+(cos30^{o}+cos150^{o})`

`=[sin45^{o}-sin(\pi-45^{o})]+[cos30^{o}+cos(\pi-30^{o})]`

`=(sin45^{o}-sin45^{o})+(cos30^{o}-cos30^{o})`

`=0`

`d)`

`D=sin60^{o}-sin120^{o}+cos45^{o}-cos135^{o}`

`=(sin60^{o}-sin120^{o})+(cos45^{o}-cos135^{o})`

`=[sin60^{o}-sin(\pi-60^{o})]+[cos45^{o}-cos(\pi-45^{o})]`

`=(sin60^{o}-sin60^{o})+(cos45^{o}-cos45^{o})`

`=0`

Bài 2:

`a)`

`A=sin23^{o}-cos(90^{o}-23^{o})`

`=sin23^{o}-sin23^{o}`

`=0`

`b)`

`B=sin(180^{o}-30^{o})+cos(90^{o}-30^{o})`

`=sin30^{o}+sin30^{o}`

`=2sin30^{o}`

`=2.(1)/(2)`

`=1`

`c)`

`C=cot(180^{o}-45^{o})+tan45^{o}`

`=-cot45^{o}+tan45^{o}`

`=-cot45^{o}+tan(\pi-45^{o})`

`=-cot45^{o}+cot45^{o}`

`=0`

`d)`

`D=tan(90^{o}-60^{o})+cot(180^{o}-120^{o})`

`=cot60^{o}+cot120^{o}`

`=cot(180^{o}-60^{o})+cot120^{o}`

`=-cot120^{o}+cot120^{o}`

`=0`