Cho hình thang vuông$ABCD$vuông ở$A$và$D$, $AD=2a$. Trên đường thẳng vuông góc tại$D$với$\left( ABCD \right)$lấy điểm$S$với$SD=a\sqrt{2}$. Tính khỏang cách giữa đường thẳng$DC$và$\left( SAB \right)$.
A.$\dfrac{a}{\sqrt{2}}$.
B.$\dfrac{a\sqrt{3}}{3}$.  
C.$\dfrac{2a}{\sqrt{3}}$.
D.$a\sqrt{2}$.    

Khoảng cách giữa hai cạnh đối trong một tứ diện đều cạnh $a$ bằng:
A.$\dfrac{a\sqrt{2}}{2}$. 
B.$\dfrac{2a}{3}$
C.$2a$.
D.$\dfrac{a\sqrt{3}}{3}$.

Cho hình lăng trụ tam giác đều $ABC.A’B’C’$ có cạnh bằng $a$ và chiều cao bằng $2a$. Gọi $M, N$ lần lượt là trung điểm của $BC$ và $A’C’$. Khoảng cách giữa hai đường thẳng $AM$ và $B'N$ bằng:
A.$ a\sqrt{2} $.
B.a.
C.2a.
D.$ a\sqrt{3} $.

Cho hình lập phương $ABCD.{A}'{B}'{C}'{D}'$ có cạnh bằng 1 (đvd). Khoảng cách giữa$A{A}'$ và $B{D}'$ bằng:
A.$\dfrac{3\sqrt{5}}{7}$  
B.$\dfrac{\sqrt{3}}{3}$                       
C.$\dfrac{\sqrt{2}}{2}$
D.$\dfrac{2\sqrt{2}}{5}$

Giá trị của $ x $ để biểu thức $ \dfrac{4\text{x}-5}{x-1}-\dfrac{x}{x-1} $ bằng $ 2 $ là
A.$ x=3 $.
B.$ x=4 $.
C.$ x=2 $.
D.$ x=5 $.

Mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to the following question.
The film…………(run) for ten minutes yet, but there's a commercial break already.
 
A.had been run
B.has not been running
C.hadn’t been ran
D.ran

Mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to the following question.
We ... for the bus for 20 minutes now, I think the bus….......
A.have been waiting - has been crashing
B.have waited - has crashed
C.have waited - has been crashing
D.have been waiting - has crashed

Mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to the following question.
The homework my teacher assigned me is endless. I ___________ it all day but still _______yet.
A.have doing/ have not been finishing
B.have done/ have not been finishing
C.have been done/ have not finished
D.have been doing/ have not finished

Mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to the following question.
It’s time Tom had to tidy his garden. It’s getting ___________.
A.more and more messy
B.more messy and messy
C.more messier and messier
D.messier and messier

Mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to the following question.
Since he (leave) ______ here no one (see) ______ him.
A.has left – saw
B.had left – saw
C.left – have seen
D.left – has seen