Cho hình lập phương ABCD.A'B'C'D' có cạnh bằng a. Khoảng cách của hai đường thẳng BD' và B'C bằng
A. a105.
B. a55.
C. a66.
D. a63.

Mệnh đề đúng trong các mệnh đề sau:
A. Hai đường thẳng cùng vuông góc với một mặt phẳng thì song song.
B. Hai đường thẳng phân biệt cùng vuông góc với một đường thẳng thì song song.
C. Hai mặt phẳng phân biệt cùng vuông góc với một đường thẳng thì song song.
D. Hai mặt phẳng phân biệt cùng vuông góc với một mặt phẳng thì song song.

Cho hàm số y = f(x) có đạo hàm trong khoảng (a; b), đồ thị là đường cong (C). Để đường thẳng Δ: y = ax + b là tiếp tuyến của (C) tại điểm M0(x0; f(x0)), điều kiện cần và đủ là:
A. a = f'(x0)
B. ax0 + b = f'(x0)
C.
D.

Choose one sentence that has the same meaning as the root one:
My father hasn't smoked for three years.
A. My father started smoking three years ago.
B. My father stopped smoking three years ago.
C. My father quit smoking for three years.
D. My father will continue smoking in three years' time.

Choose one sentence that has the same meaning as the root one:
The restaurant was often crowded, so she booked a table.
A. She booked a table in case the restaurant was crowded.
B. She booked a table even though the restaurant was crowded.
C. The restaurant was crowded; however, she booked a table.
D. The restaurant was crowded; moreover, she booked a table.

Choose one sentence that has the same meaning as the root one:
It is your duty to turn off all the electric devices.
A. You are supposed to turn off all the electric devices.
B. You are suggested to turn off all the electric devices.
C.  You are recommended to turn off all the electric devices.
D.  You are enforced to turn off all the electric devices.

Choose one sentence that has the same meaning as the root one:
My mother never allows me to go out alone at night.
A. I was not allowed to go out last night.
B. My mother never lets me go out alone at night.
C. My mother never goes out alone at night.
D. My mother and I usually go out alone at night.

Choose one sentence that has the same meaning as the root one:
You can’t make a cake without flour.
A. Unless you don’t use flour you can’t make a cake.
B. Unless you use flour you can’t make a cake.
C. You can make a cake unless you use flour.
D. You can make a cake unless you will use flour.

Choose one sentence that has the same meaning as the root one:
Don’t bother to lock the door.
A. You needn’t bother to lock the door.
B. You mustn’t bother to lock the door.
C. You can’t have bothered to lock the door.
D. You mightn’t have bothered to lock the door

Choose one sentence that has the same meaning as the root one:
“Why don’t you invite Mary to your birthday party, Jane?” said Tom.
A. Tom advised Jane to invite Mary to her birthday party.
B. Tom asked why Jane doesn't invite Mary to her birthday party.
C. Tom asked why didn’t Jane invite Mary to her birthday party.
D. Tom suggested Jane invite Mary to her birthday party.