A) 0
B) ? 7
C) 7
D) 1
Correct Answer: B
Solution :
\[\mathbf{a}+\mathbf{b}+\mathbf{c}=0\]Þ \[(\mathbf{a}+\mathbf{b}+\mathbf{c})\,.\,(\mathbf{a}+\mathbf{b}+\mathbf{c})\]= 0 Þ \[|\mathbf{a}{{|}^{2}}+|\mathbf{b}{{|}^{2}}+|\mathbf{c}{{|}^{2}}+\,2(\mathbf{a}\mathbf{.b}+\mathbf{b}\mathbf{.c}+\mathbf{c}\mathbf{.a})=0\] Þ \[\mathbf{a}\mathbf{.b}+\mathbf{b}\mathbf{.c}+\mathbf{c}\mathbf{.a}=\frac{-1-4-9}{2}=-7.\]
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