原式=[cos(30-20)-cos(30+20)]÷[sin(30-20)-sin(30+20)]=(cos30cos20+sin30sin20-cos30cos20+sin30sin20)÷(sin30cos20-cos30sin20-sin30cos20-cos30sin20)=2sin30sin20÷(-2cos30sin20)=-sin30/cos30=-√3/3