Giúp mình ví dụ 2 nhé

`(sin^3 \ B/2)/(cos ((A+2B+C)/2))-(cos^3 \ B/2)/(sin((A+2B+C)/2))`

`=(sin^3 \ B/2)/(cos \ (180^o +B)/2)-(cos^3 \ B/2)/(sin \ (180^o +B)/2)`

`=(sin^3 \ B/2)/(cos(90^o + B/2))-(cos^3 \ B/2)/(sin(90^o+B/2))`

`=(sin^3 \ B/2)/(-sin \ B/2)-(cos^3 \ B/2)/(cos \ B/2)`

`=-sin^2 \ B/2-cos^2 \ B/2`

`=-1` `(1)`

`tanA*cot(B+C)`

`=tanA*cot(180^o-A)`

`=-tanA*cotA`

`=-1` `(2)`

Từ `(1)` và `(2)` `=>` đpcm