Choose one sentence that has the same meaning as the root one:
I didn’t have an umbrella with me, so I got wet.
A. If I had had an umbrella, I wouldn’t get wet.
B. If I had had an umbrella, I would get wet.
C. If I had had an umbrella, I would have got wet.
D. If I had had an umbrella, I wouldn’t have got wet.

Choose one sentence that has the same meaning as the root one:
As soon as he arrived at the airport, he called home.
A. Calling home, he said that he had arrived at the airport.
B. No sooner had he arrived at the airport than he called home.
C. He arrived at the airport sooner than he had expected.
D. He arrived at the airport and called me to take him home.

Choose one sentence that has the same meaning as the root one:
I'm sure I told him to stay at home, but he’s gone out.
A. In spite of I told him to stay at home, he’s gone out.
B.  Despite telling him to stay at home, he’s gone out.
C. Even though I told him to stay at home, he’s gone out.
D. Although having told him to stay at home, he’s gone out.

Cho tứ diện $\displaystyle ABCD$ có$\displaystyle AC=a,BD=3a$. Gọi$\displaystyle M{{,}^{{}}}N$ lần lượt là trung điểm của$\displaystyle AD$ và$\displaystyle BC$. Biết$\displaystyle AC$vuông góc với$\displaystyle BD$. Tính$\displaystyle MN$.
A. $\displaystyle MN=\frac{a\sqrt{6}}{3}$ .
B. $\displaystyle MN=\frac{a\sqrt{10}}{2}$ .
C. $\displaystyle MN=\frac{2a\sqrt{3}}{3}$.
D. $\displaystyle MN=\frac{3a\sqrt{2}}{2}$.

Trong không gian cho hình hộp $\displaystyle ABCD.A'B'C'D'$. Khi đó 4 vectơ nào sau đây đồng phẳng?
A. $\displaystyle \overrightarrow{AC},\overrightarrow{AB},\overrightarrow{AD},\overrightarrow{AC'}$ . 
B. $\displaystyle \overrightarrow{A'D},\overrightarrow{AA'},\overrightarrow{A'D'},\overrightarrow{DD'}$.
C. $\displaystyle \overrightarrow{AC},\overrightarrow{AB},\overrightarrow{AD},\overrightarrow{AA'}\,$. 
D. $\displaystyle \overrightarrow{AB'},\overrightarrow{AB},\overrightarrow{AD},\overrightarrow{AA'}$.

Đạo hàm của hàm số y=1(x2-2x+5)  là
A. y'=-2x-2(x2-2x+5)2.
B. y'=2x+2(x2-2x+5)2.
C. y'=-2x+2(x2-2x+5).
D. y'=-4x+4(x2-2x+5)3.

Trong không gian cho ba điểm $\displaystyle A{{,}^{{}}}B{{,}^{{}}}C$ bất kỳ, chọn đẳng thức đúng?
A. $2\overrightarrow{AB}.\overrightarrow{AC}=A{{B}^{2}}+A{{C}^{2}}-B{{C}^{2}}$.
B. $2\overrightarrow{AB}.\overrightarrow{AC}=A{{B}^{2}}+A{{C}^{2}}-2B{{C}^{2}}$.
C. $\overrightarrow{AB}.\overrightarrow{AC}=A{{B}^{2}}+A{{C}^{2}}-2B{{C}^{2}}$ .
D. $\overrightarrow{AB}.\overrightarrow{AC}=A{{B}^{2}}+A{{C}^{2}}-B{{C}^{2}}$.

A.
B. 3
C.
D. Một số khác.

Cho điểm M chia đoạn thẳng AB theo tỉ số k (k ≠ 1) và O là điểm bất kì. Hệ thức đúng là
A.
B.
C.
D.

Kết quả đúng trong các kết quả sau lim(n-n+1) bằng
A. Không có giới hạn khi n→+∞.
B. -1.
C. 0.
D. Một kết quả khác.