0.5sin20-cos10 /sin3sin17- cos3cos17=√3/2.

(0.5sin20-cos10) /(sin3sin17- cos3cos17)=(0.5sin20-cos(30-20))/(-(cos3cos17-sin3sin17))=(0.5sin20-(cos30cos20+sin30sin20))/(-cos(3+17))=(0.5sin20-(√3/2cos20+1/2sin20))/(-cos20)=(1/2sin20-√3/2cos20-1/2sin20)/(-cos20)=- √3/2cos20/-cos20=√3/2