Biểu thức $A=\cos \frac{\pi }{7}.cos\frac{4\pi }{7}.cos\frac{5\pi }{7}$ có giá trị bằng
A. $\frac{1}{8}.$
B. $-\frac{1}{8}.$
C. $\frac{1}{4}.$
D. $-\frac{1}{4}.$

Tập xác định của hàm số $y=\frac{\cot x}{\cos x-1}$ là
A. $D=R\backslash \left\{ k2\pi |k\in Z \right\}.$
B. $D=R\backslash \left\{ k\frac{\pi }{2}|k\in Z \right\}.$
C. $D=R\backslash \left\{ k\pi |k\in Z \right\}.$
D. $D=R\backslash \left\{ \frac{\pi }{2}+k2\pi |k\in Z \right\}.$

Giải phương trình $\displaystyle \frac{1+{{\sin }^{2}}x}{1-{{\sin }^{2}}x}-{{\tan }^{2}}x=4$.
A. $\displaystyle x=\pm \frac{\pi }{3}+k2\pi $ 
B. $\displaystyle x=\pm \frac{\pi }{6}+k2\pi $ 
C. $\displaystyle x=\pm \frac{\pi }{3}+k\pi $ 
D. $\displaystyle x=\pm \frac{\pi }{6}+k\pi $

Giải phương trình $3{{\cos }^{4}}x-4{{\sin }^{2}}x{{\cos }^{2}}x+{{\sin }^{4}}x=0$.
A. $x=\pm \frac{\pi }{4}+k\pi $ 
B. $x=\pm \frac{\pi }{3}+k\pi $ 
C. $x=\pm \frac{\pi }{4}$ và$x=\pm \frac{\pi }{3}+k\pi $  
D. $x=\pm \frac{\pi }{4}+k2\pi $ và$x=\pm \frac{\pi }{3}+k2\pi $

Nghiệm của phương trình cos2x + cosx = 0 thỏa điều kiện:  < x < 
A. x=π
B.  
C. x =    
D.

Tìm tập giá trị lớn nhất, giá trị nhỏ nhất của hàm số sau $y=3\sin x+4\cos x+1$
A. $\max y=6$,$\min y=-2$ 
B. $\max y=4$,$\min y=-4$
C. $\max y=6$,$\min y=-4$
D. $\max y=6$,$\min y=-1$

Chu kỳ của hàm số $\displaystyle y=\cos x$ là:
A. $\displaystyle k2\pi $ 
B. $\displaystyle \frac{{2\pi }}{3}$ 
C. $\displaystyle \pi $ 
D. $\displaystyle 2\pi $

Choose the best answer for the following sentence:
I think it is a/an _______________ way to help them to read and write. 
A. sufficient                   
B. effective                  
C. honorable                 
D. pleasing

Find a mistake in the four underlined parts of the sentence and correct it:
According to a new study, every 3 months the rate of Internet use increases by 25%, that is substantial.
A. According to
B. every 3 months
C. increases by 25%
D. that

Choose the correct sentence which is built from the words and phrases given:
He / accused / of / cheated / having / was / in / his / final / exam / last / week /.
A. He was of accused having cheated in his final exam last week.
B. He was accused of having cheated in his final exam last week.
C. He was accused of having cheated in his exam final last week.
D.  He was accused of having cheated last week in his final exam.