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If a unit vector a^\hat a makes an angle π3\dfrac{\pi}{3} with i^\hat i, π4\dfrac{\pi}{4} with j^\hat j and an acute angle θ\theta with k^\hat k, then the respective value of θ\theta and a^\hat a are

Solution

✅ Correct Option: 3

Direction cosines satisfy cos⁡2α+cos⁡2β+cos⁡2γ=1\cos^2\alpha+\cos^2\beta+\cos^2\gamma = 1.

So 14+12+cos⁡2θ=1\dfrac{1}{4}+\dfrac{1}{2}+\cos^2\theta = 1, giving cos⁡θ=12\cos\theta = \dfrac{1}{2} and θ=π3\theta = \dfrac{\pi}{3} (acute).

Hence a^=12i^+12j^+12k^\hat a = \dfrac{1}{2}\hat i+\dfrac{1}{\sqrt2}\hat j+\dfrac{1}{2}\hat k.

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