\(\overrightarrow{x}\) ⊥ \(\overrightarrow{y}\)

⇒ \(\left(\overrightarrow{a}+\overrightarrow{b}\right)\left(\overrightarrow{2a}-\overrightarrow{b}\right)=0\). Đặt \(\left|\overrightarrow{a}\right|=a;\left|\overrightarrow{b}\right|=b\)

⇒ 2a² - \(\overrightarrow{a}.\overrightarrow{b}\) + 2\(\overrightarrow{a}.\overrightarrow{b}\) - b² = 0

⇒ \(\overrightarrow{a}.\overrightarrow{b}\) = b² - 2a² = 4 - 4 = 0

⇒ \(\left(\overrightarrow{a};\overrightarrow{b}\right)=90^0\)

\(\overrightarrow{u}=2\overrightarrow{a}+3\overrightarrow{b}-5\overrightarrow{c}=\left(-30;21\right)\)

A

\(2\overrightarrow{y}-\overrightarrow{z}=2\overrightarrow{a}-2\overrightarrow{b}-2\overrightarrow{c}+3\overrightarrow{b}+2\overrightarrow{c}=2\overrightarrow{a}+\overrightarrow{b}=\overrightarrow{x}\)

\(\Rightarrow\) Ba vecto \(\overrightarrow{x},\overrightarrow{y},\overrightarrow{z}\) đồng phẳng

Lời giải:

$\overrightarrow{i}=(1,0), \overrightarrow{j}=(0,1)$

$\Rightarrow \overrightarrow{i}-\overrightarrow{j}=(1-0,0-1)=(1,-1)$

Bài 2:

$\overrightarrow{a}+2\overrightarrow{b}=(3+2.-1, -4+2.2)=(1, 0)$

\(\overrightarrow{a}\perp\overrightarrow{b}\Rightarrow\overrightarrow{a}.\overrightarrow{b}=0\)

\(\left(2\overrightarrow{a}-\overrightarrow{b}\right)\left(\overrightarrow{a}+\overrightarrow{b}\right)=2a^2+2\overrightarrow{a}.\overrightarrow{b}-\overrightarrow{a}.\overrightarrow{b}-b^2\)

\(=2a^2-b^2+\overrightarrow{a}.\overrightarrow{b}\)

\(=2.1-2+0=0\)

\(\Rightarrow\left(2\overrightarrow{a}-\overrightarrow{b}\right)\perp\left(\overrightarrow{a}+\overrightarrow{b}\right)\)

a: ΔABC đều cạnh a

=>AB=AC=BC=a; \(\hat{ABC}=\hat{ACB}=\hat{BAC}=60^0\)

\(\overrightarrow{AB}\cdot\overrightarrow{AC}=AB\cdot AC\cdot cosBAC\)

\(=a\cdot a\cdot cos60=a^2\cdot\frac12=\frac{a^2}{2}\)

\(\overrightarrow{BC}\cdot\overrightarrow{AC}=\overrightarrow{CB}\cdot\overrightarrow{CA}\)

\(=CB\cdot CA\cdot cosACB\)

\(=a\cdot a\cdot cos60=a^2\cdot\frac12=\frac{a^2}{2}\)

b: \(3\cdot\overrightarrow{BM}=2\cdot\overrightarrow{BC}\)

=>\(\overrightarrow{BM}=\frac23\cdot\overrightarrow{BC}\)

=>\(BM=\frac23BC\) và M nằm giữa B và C

Ta có: BM+MC=BC

=>\(MC=BC-BM=BC-\frac23BC=\frac13BC\)

\(5\cdot\overrightarrow{AN}=4\cdot\overrightarrow{AC}\)

=>\(\overrightarrow{AN}=\frac45\cdot\overrightarrow{AC}\)

=>AN=4/5AC và N nằm giữa A và C

\(\overrightarrow{BN}\cdot\overrightarrow{AM}=\left(\overrightarrow{BA}+\overrightarrow{AN}\right)\left(\overrightarrow{AB}+\overrightarrow{BM}\right)\)

\(=\left(-\overrightarrow{AB}+\frac45\cdot\overrightarrow{AC}\right)\left(\overrightarrow{AB}+\frac23\cdot\overrightarrow{BC}\right)=-\overrightarrow{AB}\cdot\overrightarrow{AB}-\frac23\cdot\overrightarrow{AB}\cdot\overrightarrow{BC}+\frac45\cdot\overrightarrow{AC}\cdot\overrightarrow{AB}+\frac{8}{15}\cdot\overrightarrow{AC}\cdot\overrightarrow{BC}\)

\(=-AB\cdot AB\cdot cos0+\frac23\cdot\overrightarrow{BA}\cdot\overrightarrow{BC}+\frac45\cdot\overrightarrow{AB}\cdot\overrightarrow{AC}+\frac{8}{15}\overrightarrow{CA}\cdot\overrightarrow{CB}\)

\(=-AB^2+\frac23\cdot BA\cdot BC\cdot cos60+\frac45\cdot AB\cdot AC\cdot cos60+\frac{8}{15}\cdot CA\cdot CB\cdot cos60\)

\(=-a^2+\frac13a^2+\frac25a^2+\frac{4}{15}a^2=-\frac23a^2+\frac25a^2+\frac{4}{15}a^2=\frac{-10+6+4}{15}\cdot a^2=0\)

=>AM⊥BN

\(cos\left(\overrightarrow{b};\overrightarrow{a}-\overrightarrow{b}\right)=\dfrac{\overrightarrow{b}\left(\overrightarrow{a}-\overrightarrow{b}\right)}{\left|\overrightarrow{b}\right|.\left|\overrightarrow{a}-\overrightarrow{b}\right|}=\dfrac{\overrightarrow{a}.\overrightarrow{b}-\overrightarrow{b}^2}{1.\sqrt{3}}=\dfrac{2.1.cos\dfrac{\pi}{3}-1^2}{\sqrt{3}}=0\)

\(\Rightarrow\left(\overrightarrow{b};\overrightarrow{a}-\overrightarrow{b}\right)=90^0\)

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b) Ta có :

\(IB=2IC\Leftrightarrow IB=2\left(IB+BC\right)\Leftrightarrow-IB=2BC\Leftrightarrow BI=2BC\)

\(JC=-\frac{1}{2}JA\Leftrightarrow JB+BC=-\frac{1}{2}\left(JB+BA\right)\)

\(\Leftrightarrow\frac{3}{2}JB=-\frac{1}{2}BA-BC\Leftrightarrow JB=-\frac{1}{3}BA-\frac{2}{3}BC\)

\(\Rightarrow BJ=\frac{1}{3}BA+\frac{2}{3}BC\)

\(\Rightarrow IJ=BJ-BI=\frac{1}{3}BA+\frac{2}{3}BC-2BC=\frac{1}{3}BA-\frac{4}{3}BC\)

\(KA=-KB\Leftrightarrow KB+BA=-KB\Leftrightarrow2KB=-BA\)

\(\Rightarrow2BK=BA\Leftrightarrow BK=\frac{1}{2}BA\)

\(\Rightarrow JK=BK-BJ=\frac{1}{2}BA-\frac{2}{3}BC=\frac{1}{6}BA-\frac{2}{3}BC\)

\(=\frac{1}{2}\left(\frac{1}{3}BA-\frac{4}{3}BC\right)=\frac{1}{2}IJ\)

Vậy \(I,J,K\)thẳng hàng