Answer:
option A
Step-by-step explanation:
cos(4x) = cos(2x+2x) = cos(2x)*cos(2x) - sin(2x)*sin(2x)
cos(4x) = cos^2(2x) - sin^2(2x)
cos(4x) = cos^2(2x) - cos^2(2x) - 1 (using cos^2(2x) + sin^2(2x) = 1)
cos(4x) = 2cos^2(2x) - 1 (eq. 1)
cos(2x) = cos(x+x) = cos(x)*cos(x) - sin(x)*sin(x)
cos(2x) = cos^2(x) - sin^2(x)
cos(2x) = 1 - sin^2(x) - sin^2(x) (using cos^2(x) + sin^2(x) = 1)
cos(2x) = 1 - 2sin^2(x) (eq. 2)
Replacing eq. 2 into eq. 1:
cos(4x) = 2[1 - 2sin^2(x)]^2 - 1
cos(4x) = 2[1 - 4sin^2(x) + 4sin^4(x)] - 1
cos(4x) = 1 - 8sin^2(x) + 8sin^4(x)
cos(4x) - 1 + 8sin^2(x) = 8sin^4(x)
cos(4x)/4 - 1/4 + 2sin^2(x) = 2sin^4(x)
2sin^2(x)sin^2(x) = 2sin^4(x) = cos(4x)/4 - 1/4 + 2sin^2(x)
Using eq. 2:
2sin^2(x)sin^2(x) = cos(4x)/4 - 1/4 + 1 - cos(2x)
2sin^2(x)sin^2(x) = cos(4x)/4 + 3/4 - 4cos(2x)/4
2sin^2(x)sin^2(x) = [3 + cos(4x) - 4cos(2x)]/4
x 22/7
x 2/3