13. sin^2x + sin^2(2x) + sin^2(3x) = 3/2
<=> (1 - cos2x)/2 + (1 - cos4x)/2 + (1 - cos6x)/2 = 3/2
<=> 3/2 - (cos2x + cos4x + cos6x)/2 = 3/2
<=> cos2x + cos4x + cos6x = 0
<=> (cos2x + cos6x) + cos4x = 0
<=> 2cos4x.cos2x + cos4x = 0
<=> cos4x(2cos2x + 1) = 0
<=> cos4x = 0 hoặc cos2x = -1/2 = cos2π/3
<=> 4x = π/2 + kπ
hoặc 2x = 2π/3 + k2π hoặc 2x = -2π/3 + k2π, k nguyên
<=> x = π/8 + kπ/4
hoặc x = π/3 + kπ hoặc x = -π/3 + kπ, k nguyên

14. cos(π/7 - 3x) = -√3 /2
<=> cos(π/7 - 3x) = cos5π/6
<=> π/7 - 3x = 5π/6 + k2π
hoặc π/7 - 3x = -5π/6 + k2π, k nguyên
<=> 3x = π/7 - 5π/6 - k2π
hoặc 3x = π/7 + 5π/6 - k2π, k nguyên
<=> x = -29π/126 - k2π/3
hoặc x = 41π/126 - k2π/3 , k nguyên
15. 2cos^2x - sin^2x - 4cosx + 2 = 0
<=> 2cos^2x + cos^2x - 4cosx + 1 = 0
<=> 3cos^2x - 4cosx + 1 = 0
<=> (cosx - 1)(3cosx - 1) = 0
<=> cosx = 1 hoặc cosx = 1/3
<=> x = k2π hoặc x = arccos1/3 + k2π
hoặc x = -arccos1/3 + k2π, k nguyên