Let $\vec{a}$ and $\vec{b}$ be two vectors such that $|\vec{a}|=1,|\vec{b}|=4$, and $\vec{a} \cdot \vec{b}=2$. If $\vec{c}=(2 \vec{a} \times \vec{b})-3 \vec{b}$ and the angle between $\vec{b}$ and $\vec{c}$ is $\alpha$, then $192 \sin ^2 \alpha$ is equal to ________.
Answer (integer)
48
Solution
<p>$$\begin{aligned}
& \overrightarrow{\mathrm{b}} \cdot \overrightarrow{\mathrm{c}}=(2 \overrightarrow{\mathrm{a}} \times \overrightarrow{\mathrm{b}}) \cdot \overrightarrow{\mathrm{b}}-3|\mathrm{b}|^2 \\
& |\mathrm{~b}||c| \cos \alpha=-3|\mathrm{~b}|^2 \\
& |\mathrm{c}| \cos \alpha=-12 \text {, as }|\mathrm{b}|=4 \\
& \overrightarrow{\mathrm{a}} \cdot \overrightarrow{\mathrm{b}}=2 \\
& \cos \theta=\frac{1}{2} \Rightarrow \theta=\frac{\pi}{3} \\
& |c|^2=|(2 \overrightarrow{\mathrm{a}} \times \overrightarrow{\mathrm{b}})-3 \overrightarrow{\mathrm{b}}|^2 \\
& =64 \times \frac{3}{4}+144=192 \\
& |c|^2 \cos ^2 \alpha=144 \\
& 192 \cos ^2 \alpha=144 \\
& 192 \sin ^2 \alpha=48
\end{aligned}$$</p>
About this question
Subject: Mathematics · Chapter: Vector Algebra · Topic: Types of Vectors
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