TRIGONOMETRY

sin2x + sin3x = 0

2sin(5x/2) cos(x/2) = 0sin(5x/2) = 0 or cos(x/2) = 05x/2 = 0, 180, 360,

x/2 = 90,

So, x = 0,72,144,180,

2 cos3θ

= cos(2θ + θ)

= cos2θcosθ - sin2θsinθ

= cosθ(2cos^2θ - 1) - 2sinθ^2 cosθ

= cosθ(2cos^2θ - 1) - 2(1 - cosθ^2) cosθ

= 4cos^3θ - 3cosθ