Learn Derivatives of sin x, cos x, tan x, e^x and ln x | Chain rule | Khan Academy
Once you say that "x = some number", x is no longer a *variable* but rather it is a *constant* -- and you must then treat it like a constant. Since x = π/6, both "x" and "sin(x)" are constants (because sin(π/6) = 1/2) and the derivative of a constant is z...