MHT CET · Maths · Vector Algebra
Let \(\overline{\mathrm{a}}, \overline{\mathrm{b}}\) and \(\overline{\mathrm{c}}\) be three non-zero vectors such that no two of them are collinear and \(\left.(\overline{\mathrm{a}} \times \overline{\mathrm{b}}) \times \overline{\mathrm{c}}=\frac{1}{3}|\overline{\mathrm{~b}}| \overline{\mathrm{c}} \right\rvert\, \overline{\mathrm{a}}\). If \(\theta\) is the angle between vectors \(\bar{b}\) and \(\bar{c}\), then the value of \(\sin \theta\) is
- A \(\frac{2}{3}\)
- B \(\frac{-2 \sqrt{2}}{3}\)
- C \(\frac{2 \sqrt{2}}{3}\)
- D \(\frac{-\sqrt{2}}{3}\)
Answer & Solution
Correct Answer
(C) \(\frac{2 \sqrt{2}}{3}\)
Step-by-step Solution
Detailed explanation
Given, \((\overline{\mathrm{a}} \times \overline{\mathrm{b}}) \times \overline{\mathrm{c}}=\frac{1}{3}|\overline{\mathrm{~b}}||\overline{\mathrm{c}}| \overline{\mathrm{a}}\)
We know that,
\((\overline{\mathrm{a}} \times \overline{\mathrm{b}}) \times \overline{\mathrm{c}}=(\overline{\mathrm{a}} \cdot \overline{\mathrm{c}}) \overline{\mathrm{b}}-(\overline{\mathrm{b}} \cdot \overline{\mathrm{c}}) \overline{\mathrm{a}}\)
On comparing, we get
\(\begin{aligned}
& \frac{1}{3}|\overline{\mathrm{~b}}||\overline{\mathrm{c}}|=-\overline{\mathrm{b}} \cdot \overline{\mathrm{c}} \\
& \Rightarrow \frac{1}{3}|\overline{\mathrm{~b}}||\overline{\mathrm{c}}|=-|\overline{\mathrm{b}}||\overrightarrow{\mathrm{c}}| \cos \theta \\
& \Rightarrow \cos \theta=\frac{-1}{3} \\
& \Rightarrow \cos ^2 \theta=\frac{1}{9}
\end{aligned}\)
\(\begin{aligned} \sin ^2 \theta & =1-\cos ^2 \theta \\ & =1-\frac{1}{9} \\ \therefore \quad \sin ^2 \theta & =\frac{8}{9} \\ \therefore \quad \sin \theta & =\sqrt{\frac{8}{9}}=\frac{2 \sqrt{2}}{3}\end{aligned}\)
We know that,
\((\overline{\mathrm{a}} \times \overline{\mathrm{b}}) \times \overline{\mathrm{c}}=(\overline{\mathrm{a}} \cdot \overline{\mathrm{c}}) \overline{\mathrm{b}}-(\overline{\mathrm{b}} \cdot \overline{\mathrm{c}}) \overline{\mathrm{a}}\)
On comparing, we get
\(\begin{aligned}
& \frac{1}{3}|\overline{\mathrm{~b}}||\overline{\mathrm{c}}|=-\overline{\mathrm{b}} \cdot \overline{\mathrm{c}} \\
& \Rightarrow \frac{1}{3}|\overline{\mathrm{~b}}||\overline{\mathrm{c}}|=-|\overline{\mathrm{b}}||\overrightarrow{\mathrm{c}}| \cos \theta \\
& \Rightarrow \cos \theta=\frac{-1}{3} \\
& \Rightarrow \cos ^2 \theta=\frac{1}{9}
\end{aligned}\)
\(\begin{aligned} \sin ^2 \theta & =1-\cos ^2 \theta \\ & =1-\frac{1}{9} \\ \therefore \quad \sin ^2 \theta & =\frac{8}{9} \\ \therefore \quad \sin \theta & =\sqrt{\frac{8}{9}}=\frac{2 \sqrt{2}}{3}\end{aligned}\)
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