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A line makes angle θ\theta with xx-axes as well as zz-axis. If the angle β\beta, which it makes with yy-axis is such that sin⁡2β=3sin⁡2θ\sin^2\beta = 3\sin^2\theta, then cos⁡2θ\cos^2\theta is :

Solution

✅ Correct Option: 3

cos⁡2θ+cos⁡2β+cos⁡2θ=1\cos^2\theta + \cos^2\beta + \cos^2\theta = 1, so cos⁡2β=1−2cos⁡2θ\cos^2\beta = 1 - 2\cos^2\theta. Then sin⁡2β=2cos⁡2θ=3sin⁡2θ=3(1−cos⁡2θ)\sin^2\beta = 2\cos^2\theta = 3\sin^2\theta = 3(1-\cos^2\theta). So 2cos⁡2θ=3−3cos⁡2θ2\cos^2\theta = 3 - 3\cos^2\theta, 5cos⁡2θ=35\cos^2\theta = 3, cos⁡2θ=35\cos^2\theta = \frac{3}{5}.

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