Gấp ạ!!!!!!!!!!!!!!!!!

Đáp án+Giải thích các bước giải:

a,

`cos165^@`

`=cos(120^@+45^@)`

`=cos120^@. cos45^@-sin120^@ .sin45^@`

`=-1/2 .\sqrt{2}/2-\sqrt{3}/2. \sqrt{2}/2`

`=-\sqrt{2}/4-\sqrt{6}/4`

`=-(\sqrt{2}+\sqrt{6})/4`

b,

`tan((7\pi)/12)`

`=tan(\pi/4+\pi/3)`

`=[tan(\pi/4)+tan(\pi/3)]/[1-tan(\pi/4).tan(\pi/3)]`

`=(1+\sqrt{3})/(1-1.\sqrt{3})`

`=(\sqrt{3}+1)^2/[1-\sqrt{3})(1+\sqrt{3})`

`=(3+1+2\sqrt{3})/(1-3)`

`=(4+2\sqrt{3})/-2`

`=-2-\sqrt{3}`