Mark M. answered 09/26/17
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Retired math prof. Calc 1, 2 and AP Calculus tutoring experience.
By direct substitution the given limit has the indeterminate form 0/0.
So, using L'Hopital's Rule, the given limit is equivalent to:
limx→π/6 [(-sinx - √3 cosx) / 6] = (-sin(π/6) - √3 cos(π/6))/6
= (-1/2 - 3/2) / 6 = -1/3