sina=-1/2
又 sin²a+cos²a=1
從而 cosa=±√3/2
∴原式=1/cos(a-7π)=1/(-cosa)=±2√3/3?????????????m^2+2n+2p^2-2mn-2np-6p+9=0?????????????? ???? m^2+2n^2+2p^2-2mn-2np-6p+9=0?? m^2-2mn+n^2+n^2+2p^2-2np-6p+9=0 (m-n)^2+n^2-2np+p^2+p^2-6p+9=0 (m-n)^2++(n-p)^2+(p-3)^2=0??? m-n=0 n-p=0 p-3=0?? m=n=p=3????n^2??p??/m??? =????p^2??p??/p =p^2 =3^2 =9