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← JEE Advanced 2013 · Paper 1
MathematicsSingle correct+2Question 44 of 60

JEE Advanced 2013 · Paper 1, Mathematics question 44

The number of points in (−∞,∞)(-\infty,\infty), for which x2−xsin⁡x−cos⁡x=0x^2-x\sin x-\cos x=0, is

Choose one option
Answer and worked solution

Official answer: C

Let f(x)=x2−xsin⁡x−cos⁡xf(x)=x^2-x\sin x-\cos x, an even function with f(0)=−1f(0)=-1.

Monotone. f′(x)=2x−sin⁡x−xcos⁡x+sin⁡x=x(2−cos⁡x)f'(x)=2x-\sin x-x\cos x+\sin x=x(2-\cos x), positive for x>0x>0. So ff increases from −1 to ∞ on (0,∞)(0,\infty): one root; by symmetry another for x<0x<0.

Two points.

(C)