sin(π／6－θ）=﹣3／5,π／6＜θ＜2π／3,
则cos(π／6－θ）=4／5,
sinθ=sin[π／6 -(π／6－θ)]
= sinπ／6cos(π／6－θ)- cosπ／6 sin(π／6－θ）
=(4+3√3)/10.
cos2θ=1-2 sin²θ=(7-24√3)/50.