Aspire Faculty ID #16196 · Topic: NIMCET 2011 · Just now
NIMCET 2011

$ \vec{a} = x\hat{i} - 3\hat{j} - \hat{k},\quad \vec{b} = 2x\hat{i} + x\hat{j} - \hat{k} $ Angle between $ \vec{a} $ and $ \vec{b} $ is acute and angle between $ \vec{b} $ and $ +y $ axis lies in $ \left(\dfrac{\pi}{2}, \pi\right) $ Find $x$.

Solution

$ \vec{a}\cdot\vec{b} = 2x^{2} - 3x + 1 > 0 \Rightarrow x < \frac12 \text{ or } x > 1 $ Angle with $+y$ axis obtuse: $ \cos\theta = \dfrac{x}{\sqrt{5x^{2}+1}} < 0 \Rightarrow x < 0 $ Combined: $ x < 0 $

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