2x=π/3+2kπ or 2X=-π/3+2kπ.

{x|x=π/6+kπ or x =-π/3+kπ, and k belongs to R}

2. Is it quadratic?

A-c = (the root number of cos (3x/2) is three times, -sin (3x/2).

f(x)=3[cos(3x/2)]^2+[-sin(3x/2)]^2=2[cos(3x/2)]^2+ 1=cos(3x)+2

Increase the interval: -π+2kπ

-π/3+2kπ/3 & lt； x & ltπ/3+2kπ/3

[-π/3+2kπ/3, π/3+2kπ/3]k belongs to R.