CTV(13π/12)-ctg(15π/12)

Question

CTV(13π/12)-ctg(15π/12)

Answer 1

(сos13π/12*sin15π/12-cos15π/12*sin13π/12)/sin15π/12*sin13π/12=
sin(15π/12-13π/12)/sin15π/12*sin13π/12=sinπ/6:1/2(cosπ/6-cos7π/6)=
1/(cosπ/6+cosπ/6)=1:2cosπ/6=1:2*√3/2=-/√3