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![](https://rs.olm.vn/images/avt/0.png?1311)
Gọi \(M\left(x;0\right)\Rightarrow\left\{{}\begin{matrix}\overrightarrow{MA}=\left(-1-x;4\right)\\\overrightarrow{MB}=\left(1-x;-2\right)\end{matrix}\right.\) \(\Rightarrow\overrightarrow{MA}+2\overrightarrow{MB}=\left(1-3x;0\right)\)
\(\Rightarrow\left|\overrightarrow{MA}+2\overrightarrow{MB}\right|=\sqrt{\left(1-3x\right)^2}\ge0\)
Dấu "=" xảy ra khi \(x=\frac{1}{3}\Rightarrow M\left(\frac{1}{3};0\right)\)
Gọi \(P\left(0;y\right)\) \(\Rightarrow\left\{{}\begin{matrix}\overrightarrow{PA}=\left(-1;4-y\right)\\\overrightarrow{PB}=\left(1;-2-y\right)\\\overrightarrow{PC}=\left(3;4-y\right)\end{matrix}\right.\)
\(\Rightarrow\overrightarrow{PA}+2\overrightarrow{PB}-4\overrightarrow{PC}=\left(-11;5y-16\right)\)
\(\Rightarrow\left|\overrightarrow{PA}+\overrightarrow{PB}-4\overrightarrow{PC}\right|=\sqrt{11^2+\left(5y-16\right)^2}\ge11\)
Dấu "=" xảy ra khi \(5y-16=0\Rightarrow y=\frac{16}{5}\Rightarrow P\left(0;\frac{16}{5}\right)\)
![](https://rs.olm.vn/images/avt/0.png?1311)
a/ Gọi K (hay L gì đó) có tọa độ \(K\left(0;y\right)\)
\(\Rightarrow\left\{{}\begin{matrix}\overrightarrow{AB}=\left(4;3\right)\\\overrightarrow{CK}=\left(-5;y-10\right)\end{matrix}\right.\)
Do AB//CK \(\Leftrightarrow\frac{-5}{4}=\frac{y-10}{3}\Rightarrow y=\frac{25}{4}\) \(\Rightarrow K\left(0;\frac{25}{4}\right)\)
b/ Gọi \(J\left(x;0\right)\Rightarrow\overrightarrow{JA}=\left(-1-x;2\right)\) ; \(\overrightarrow{JB}=\left(3-x;5\right)\); \(\overrightarrow{JC}=\left(5-x;10\right)\)
\(\Rightarrow\overrightarrow{JA}-2\overrightarrow{JB}+4\overrightarrow{JC}=\left(13-3x;32\right)\)
\(\Rightarrow T=\left|\overrightarrow{JA}-2\overrightarrow{JB}+4\overrightarrow{JC}\right|=\sqrt{\left(13-3x\right)^2+32^2}\ge32\)
\(T_{min}=32\) khi \(13-3x=0\Leftrightarrow x=\frac{13}{3}\Rightarrow J\left(\frac{13}{3};0\right)\)
c/ Gọi \(Q\left(0;y\right)\Rightarrow\left\{{}\begin{matrix}\overrightarrow{AQ}=\left(1;y-2\right)\\\overrightarrow{QC}=\left(5;10-y\right)\end{matrix}\right.\)
\(\Rightarrow T=AQ+CQ=\sqrt{1^2+\left(y-2\right)^2}+\sqrt{5^2+\left(10-y\right)^2}\)
\(\Rightarrow T\ge\sqrt{\left(1+5\right)^2+\left(y-2+10-y\right)^2}=10\)
\(T_{min}=10\) khi \(\frac{y-2}{1}=\frac{10-y}{5}\Leftrightarrow y=\frac{10}{3}\Rightarrow Q\left(0;\frac{10}{3}\right)\)
d/ Gọi \(P\left(x;0\right)\Rightarrow\left\{{}\begin{matrix}\overrightarrow{AP}=\left(x+1;-2\right)\\\overrightarrow{PB}=\left(3-x;5\right)\end{matrix}\right.\)
\(\Rightarrow T=PA+PB=\sqrt{\left(x+1\right)^2+\left(-2\right)^2}+\sqrt{\left(3-x\right)^2+5^2}\)
\(\Rightarrow T\ge\sqrt{\left(x+1+3-x\right)^2+\left(-2+5\right)^2}=5\)
\(T_{min}=5\) khi \(\frac{x+1}{-2}=\frac{3-x}{5}\Rightarrow x=-\frac{11}{3}\Rightarrow P\left(-\frac{11}{3};0\right)\)
![](https://rs.olm.vn/images/avt/0.png?1311)
- Gọi tọa độ điểm P ( x; y )
\(\Rightarrow\left\{{}\begin{matrix}\overrightarrow{PA}=\left(1-x;4-y\right)\\\overrightarrow{PB}=\left(6-x;-1-y\right)\end{matrix}\right.\)
Mà \(\overrightarrow{PA}=\dfrac{1}{3}\overrightarrow{PB}\)
\(\Rightarrow\left\{{}\begin{matrix}1-x=\dfrac{1}{3}\left(6-x\right)\\4-y=\dfrac{1}{3}\left(-1-y\right)\end{matrix}\right.\)
\(\Rightarrow\left\{{}\begin{matrix}x=-\dfrac{3}{2}\\y=\dfrac{13}{2}\end{matrix}\right.\)
Vậy tọa độ của điểm P thỏa mãn là : \(P\left(-\dfrac{3}{2};\dfrac{13}{2}\right)\)
![](https://rs.olm.vn/images/avt/0.png?1311)
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