f(x)=2sinx + cosx - ctgx
f'(x)=2cosx - sinx - 1/sin²x
f'(π/6)=2·cos(π/6) - sin(π/6) - 1/sin²(π/6) =
= 2·0.5√3 - 0.5 - 1/(1/4) = √3 - 0.5 - 4 = √3 - 4.5
Примерно: √3 - 4.5 ≈ 1,73 - 4,5 ≈ - 2,77
