Cos x is negative in (π/2 , π) . So near 2π/3 on both sides, at 2π/3 +- delta
Sin x is positive. in this domain.
so in this domain y = - cos x + sin x
differentiating dy/dx = sin x + cos x
At 2π/3 , dy/dx is sin 2π/3 + cos 2π/3 = √3/2 - 1/2 = (√3 -1 )/2
