4.2.42. f'(x) = (cosx)' - (2x)' = -sinx - 2
f'(pi/6) = -sinpi/6 - 2 = -1/2 - 2 = -2 1/2 = -2,5
4.2.43. f'(x) = (tgx)' + (pi)' = 1/cos^2x
f'(3pi/4) = 1/cos^2(3pi/4) = 1/(1/2) = 2
4.2.44. f'(x) = (ctgx)' + (3x)' + (8)' = 1/sin^2x + 3
f'(-pi/6) = 1/sin^2(-pi/6) + 3 = 1/(1/4) + 3 = 4 + 3 = 7