因为a为钝角而sin(a+π/4）>0所以a+π/4还是钝角所以cos(a+π/4)<0所以cos(a+π/4)=-√(1-1/9)=-2√2/3sin{π/2-(a+π/4)}=cos(a+π/4)=-√(1-1/9)=-2√2/3sin(π/4-a)=sin{π/2-(a+π/4)}=cos(a+π/4)=-√(1-1/9)=-2√2/3
