Đáp án:
Câu 17: B
Câu 18: B
Câu 21: B
Giải thích các bước giải:
\(\begin{array}{l}
Cau\,\,17:\\
A \in Ox \Rightarrow A\left( {a;\,\,0} \right)\\
B \in Oy \Rightarrow B\left( {0;\,\,b} \right)\\
K\left( {1;\,\, - 3} \right)\\
\Rightarrow A\,\,la\,\,trung\,diem\,\,cua\,\,KB\\
\Rightarrow \left\{ \begin{array}{l}
2a = 1 + 0\\
2.0 = b - 3
\end{array} \right. \Leftrightarrow \left\{ \begin{array}{l}
a = \frac{1}{2}\\
b = 3
\end{array} \right. \Rightarrow \left\{ \begin{array}{l}
A\left( {\frac{1}{2};\,\,0} \right)\\
B\left( {0;\,\,3} \right)
\end{array} \right..\\
\Rightarrow Chon\,\,\,A.\\
Cau\,\,18:\\
Goi\,\,\,D\left( {a;\,\,b} \right)\\
ABDC\,\,\,la\,\,\,hbh \Rightarrow \overrightarrow {AB} = \overrightarrow {CD} \\
\Leftrightarrow \left( { - 7;\,\,3} \right) = \left( {a - 4;\,\,b - 3} \right)\\
\Leftrightarrow \left\{ \begin{array}{l}
a - 4 = - 7\\
b - 3 = 3
\end{array} \right. \Leftrightarrow \left\{ \begin{array}{l}
a = - 3\\
b = 6
\end{array} \right. \Rightarrow D\left( { - 3;\,\,6} \right).\\
\Rightarrow Chon\,\,B/
\end{array}\)
\(\begin{array}{l}
Cau\,\,21:\\
\overrightarrow a = \left( {2016\sqrt {2015} ;\,\,\,0} \right);\,\,\,\overrightarrow b = \left( {4;\,\,x} \right)\\
\overrightarrow a \,\,cung\,\,phuong\,\,voi\,\,\overrightarrow b \\
\Leftrightarrow \overrightarrow b = k\overrightarrow a \\
\Leftrightarrow x = 0.
\end{array}\)
