解，cos(兀/4-阝)=cos(阝-兀/4) cos(兀/4)=cos(-兀/4)=cos(2k兀+兀/4) =cos(2k兀-兀/4)而cos(兀/4-阝)=cos(兀/4)则阝-兀/4=2K兀土兀/4
