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JEE Main 2018
Trigonometric Equations
Trigonometric Equations
Hard

Question

The number of solution of tanx+secx=2cosx\tan \,x + \sec \,x = 2\cos \,x in [0,2π]\left[ {0,\,2\,\pi } \right] is

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Solution

We can simplify the equation by converting everything to sines and cosines: sinxcosx+1cosx=2cosx\frac{\sin x}{\cos x} + \frac{1}{\cos x} = 2\cos x Multiplying through by cosx\cos x gives: sinx+1=2cos2x\sin x + 1 = 2\cos^2 x Using the identity cos2x+sin2x=1\cos^2 x + \sin^2 x = 1, we can substitute sin2x\sin^2 x with 1cos2x1 - \cos^2 x to get: sinx+1=22sin2x\sin x + 1 = 2 - 2\sin^2 x Rearranging terms gives: 2sin2x+sinx1=02\sin^2 x + \sin x - 1 = 0 This is a quadratic equation in sinx\sin x, which we can solve using the quadratic formula: sinx=1±1+84\sin x = \frac{-1 \pm \sqrt{1 + 8}}{4} The discriminant is positive, so there are two solutions for sinx\sin x: sinx=1±94=1,12\sin x = \frac{-1 \pm \sqrt{9}}{4} = -1, \frac{1}{2} For sinx=1\sin x = -1, we have x=3π2x = \frac{3\pi}{2}. For sinx=12\sin x = \frac{1}{2}, we have x=π6,5π6x = \frac{\pi}{6}, \frac{5\pi}{6}. Therefore, there are a total of 3\boxed{3} solutions in the interval [0,2π]\left[ 0, 2\pi \right].

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