The correct option (d) 7
Explanation:
∵ cosx + cos2x + cos3x + cos4x = 0
∴ cosx + cos3x + cos2x + cos4x = 0
∴ 2cos2x ∙ cosx + 2cos3x ∙ cosx = 0
∴ 2cosx(cos2x + cos3x) = 0
∴ 4cosx ∙ cos(5x/2) ∙ cos(x/2) = 0
∴ x = (π/2), (3π/2), (π/5), (3π/5), (7π/5), (9π/5) & π
∴ Total solution = 7.
