11. Given $\cos ^{2} \alpha-\cos ^{2} \beta=m, \sin (\alpha+\beta) \sin (\alpha-\beta)=$
11. $-m \cdot \sin (\alpha+\beta) \sin (\alpha-\beta)=\frac{1}{2}[\cos 2 \beta-\cos 2 \alpha]=\cos ^{2} \beta-\cos ^{2} \alpha=-m$.