# Basic Maths Grand Test- Q13

Which of the above are CORRECT ?

A. Only (A)

B. (A) and (C)

C. (B) and (C)

D. (B) and (D)

reshown Nov 20, 2019

+1 vote

$f(x)=cos2x+cos4x$

$f^{'}(x)=0$

$(-sin2x*2)-(sin4x*4) =0$

$cos2x=\frac{-1}{4}$

As you can see, there are 4 points in $(-\Pi,\Pi)$ where $cos2x=\frac{-1}{4}$

And also f(x)=cos2x+cos4x attains its maximum value at x=0.

So option C.(B) and (C) is correct.

answered Nov 20, 2019 by (2,920 points)
edited Nov 21, 2019 by
cos2x = -1/4