5π/12=75°

10cos5π/12/√6−√2=10*(cos(45°+30°))/√6−√2

cos(45°+30°)=сos45°cos30°-sin45°sin30°=√3*√2/4-√2/4=√2*(√3-1)/4

10*(cos(45°+30°))/√6−√2=2.5(√3-1)/√3-√2=2.5-2.5/√3-√2