Đáp án: chọn D
Giải thích các bước giải:
`(cos^2 x - sin^2 y)/(sin^2 x. sin^2 y) - cot^2 x . cot^2 y`
`= (cos^2 x - sin^2 y)/(sin^2 x. sin^2 y) - ((cosx.cosy)/(sin x.siny))^2 `
`=(cos^2 x - sin^2 y)/(sin^2 x. sin^2 y) - (cos^2 x.cos^2y)/(sin^2 x.sin^2 y)`
`= (cos^2x - sin^2y - cos^2 x.cos^2 y)/(sin^2 x .sin^2 y)`
`= (cos^2x (1- cos^2 y) -sin^2y)/(sin^2 x .sin^2 y)`
`=(sin^2 y .cos^2 -sin^2 y)/(sin^2 x .sin^2 y)`
`= ((cos^2 x -1).sin^2 y)/(sin^2 x .sin^2 y)`
`= (-sin^2 x.sin y)/(sin^2 x .sin^2 y)`
`= -1`
