Explanation
If a line makes an angle \alpha , \beta , \gamma with the axes, then
\cos^{2}\alpha +\cos^{2}\beta +\cos^{2}\gamma=1 [\because \cos \alpha,\cos \beta,\cos \gamma are directional cosines ]
3-(\sin^{2}\alpha +\sin^{2}\beta +\sin^{2}\gamma)=1
\Rightarrow \ \sin^{2}\alpha +\sin^{2}\beta +\sin^{2}\gamma=2
Also \sin^{2}\alpha +\sin^{2}\beta +\sin^{2}\gamma \ \ge \sin \alpha \sin \beta +\sin \beta \sin \gamma +\sin \gamma \sin \alpha
\Rightarrow \displaystyle \sum \sin \alpha \sin \beta \le 2 \dots(1)
Also (\displaystyle \sum \sin \alpha)^{2}=\displaystyle \sum \sin^{2}\alpha +2\displaystyle \sum \sin \alpha \sin \beta [\because (a+b+c)^2=a^2+b^2+c^2+2ab+2bc+2ca]
(\displaystyle \sum \sin \alpha)^{2} > 0
\displaystyle \sum \sin^{2}\alpha +2\displaystyle \sum \sin \alpha \sin \beta > 0
2+2\displaystyle \sum \sin \alpha \sin \beta > 0
2\displaystyle \sum \sin \alpha \sin \beta > -2
\displaystyle \sum \sin \alpha \sin \beta > \dfrac {-2}{2}
\displaystyle \sum \sin \alpha \sin \beta >-1 \dots(2)
From (1)\ and\ (2)
\displaystyle 2 \geq \sum \sin \alpha \sin \beta >-1
Hence the range of \displaystyle \sum \sin \alpha \sin \beta is (-1, 2]
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