A) \[{{30}^{o}}\]
B) \[{{45}^{o}}\]
C) \[{{60}^{o}}\]
D) \[{{90}^{\text{o}}}\]
Correct Answer: C
Solution :
Given, \[\mathbf{a}+\mathbf{b}+\mathbf{c}=0\Rightarrow \mathbf{a}+\mathbf{b}=-\mathbf{c}\] Squaring on both sides, Þ \[|\mathbf{a}{{|}^{2}}+|\mathbf{b}{{|}^{2}}+2|\mathbf{a}||\mathbf{b}|\cos \theta =|-\mathbf{c}{{|}^{2}}\] \[\Rightarrow 9+25+30\cos \theta =49\] Þ \[\cos \theta =\frac{1}{2}\Rightarrow \theta =60{}^\circ \].
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