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The angle between the two planes x+y−z=3x + y - z = 3 and 3x+2y+z=53x + 2y + z = 5 is :

Solution

✅ Correct Option: 2

Normals: n1⃗=(1,1,−1)\vec{n_1} = (1,1,-1), n2⃗=(3,2,1)\vec{n_2} = (3,2,1). cos⁡θ=n1⃗⋅n2⃗∣n1⃗∣∣n2⃗∣=3+2−1314=442=24221\cos\theta = \frac{\vec{n_1} \cdot \vec{n_2}}{|\vec{n_1}||\vec{n_2}|} = \frac{3+2-1}{\sqrt{3}\sqrt{14}} = \frac{4}{\sqrt{42}} = \frac{2\sqrt{42}}{21}. So θ=cos⁡−124221\theta = \cos^{-1}\frac{2\sqrt{42}}{21}.

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