TÌM TXĐ:

a. y=\(\dfrac{cos2x}{1-sin2x}\)

b. y= cosx- tan(4x+\(\dfrac{\pi}{3}\)) +5

c. y=cos\(\dfrac{x}{3}\)- \(\dfrac{3}{1+sin2x}\)+ \(\dfrac{2}{3}\)

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