18x^2 + 3x + 12x + 2 = 18x^2 - 2x + 45x - 5 - 3x
15x + 2 = 40x - 5
-25x = -7
x = 7/25 = 0.28
18) f '(x) = (-2)' * ctgx - (-2)* (ctgx)' = 0 * ctgx + 2 * (- 1/Sin²x) = - 2/Sin²x
19) f ' (x) = (Sin15x)' = Cos15x * (15x)' = 15Cos15x
20) f '(x) = [Cos(π/4 - 12x)] ' = - Sin(π/4 - 12x) * (π/4 - 12x)' = 12Sin(π/4 - 12x)