Aspire Faculty ID #16445 · Topic: NIMCET 2009 · Just now
NIMCET 2009

If $\vec{a}, \vec{b}, \vec{c}$ are non-coplanar unit vectors and $\vec{a} \times (\vec{b} \times \vec{c}) = \dfrac{\vec{b} + \vec{c}}{\sqrt{2}}$, then the angle between $\vec{a}$ and $\vec{b}$ is:

Solution

Vector triple product identity: 
$\vec{a} \times (\vec{b} \times \vec{c}) = (\vec{a}\cdot \vec{c})\vec{b} - (\vec{a}\cdot \vec{b})\vec{c}$ 
Given equals $\dfrac{\vec{b} + \vec{c}}{\sqrt{2}}$ 
So compare coefficients: 
 $\vec{b}$ coefficient: 
$a\cdot c = \dfrac{1}{\sqrt{2}}$ $\vec{c}$ coefficient: $-(a\cdot b) = \dfrac{1}{\sqrt{2}}$ $\Rightarrow a\cdot b = -\dfrac{1}{\sqrt{2}}$ Thus angle $\theta$ between $a$ and $b$ satisfies: $\cos\theta = -\dfrac{1}{\sqrt{2}}$ $\Rightarrow \theta = \dfrac{3\pi}{4}$

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