1, y= 5tan4x – 3x 2, y= tanx – 3cotx 3, y = (x^2 + 3)tanx + 1/x

1.

Để $\tan4x$ xác định:

ĐK: $\cos 4x\ne 0$

$\Leftrightarrow 4x\ne \dfrac{\pi}{2}+k\pi$

$\Leftrightarrow x\ne\dfrac{\pi}{8}+\dfrac{k\pi}{4}$

2.

Để $\tan x$, $\cot x$ xác định:

ĐK: $\cos x\ne 0; \sin x\ne 0$

$\Leftrightarrow \sin2x\ne 0$

$\Leftrightarrow 2x\ne k\pi$

$\Leftrightarrow x\ne \dfrac{k\pi}{2}$

3.

ĐK: $\cos x\ne 0; x\ne 0$

$\Leftrightarrow x\ne \dfrac{\pi}{2}+k\pi; x\ne 0$