Let f(x)=4cos3x+33cos2x−10. The number of points of local maxima of f in interval ( 0,2π ) is:
(A)1
(B)2✓ Correct
(C)3
(D)4
Answer: (B)
Solution
f(x)=4cos3x+33cos2x−10, so f′(x)=−12cos2xsinx−63cosxsinx=−6sinxcosx(2cosx+3). On (0,2π) the zeros are x=2π,π,23π and cosx=−23⇒x=65π,67π. Sign analysis shows local maxima at x=65π and x=67π — two local maxima. Option (2).
JEE Main · 2024 · 08 Apr 2024 · Shift 1 · Single correct option