giải pt: |cos x|=sin x

`|cos x| = sin x`

`<=>` $\left[\begin{matrix} cos x = sin x\\ cos x = - sin x\end{matrix}\right.$

`\text{TH1:}` `cos x = sin x`

`<=> cos x = cos (\frac{\pi}{2} - x)`

`<=>` $\left[\begin{matrix} x = \frac{\pi}{2} - x + k.2\pi\\ x = x - \frac{\pi}{2} + k.2\pi\end{matrix}\right.$ `(k in ZZ)`

`<=> x = \frac{\pi}{4} + k.\pi` `(k in ZZ)` `(1)`

`\text{TH2:}` `cos x = - sin x`

`<=> cos x + sin x = 0`

`<=> 1 + tan x = 0`

`<=> tan x = - 1`

`<=> x = - \frac{\pi}{4} + k.\pi` `(k in ZZ)` `(2)` 

`\text{Từ (1) và (2)}` `=> x = \frac{\pi}{4} + \frac{k.\pi}{2}` `(k in ZZ)`

` Vậy ` ______________________________________________

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