A) \[~{{30}^{o}}\]
B) \[~{{90}^{o}}\]
C) \[~{{60}^{o}}\]
D) \[{{45}^{o}}\]
Correct Answer: B
Solution :
The resultant of the first two should be equal and opposite to the third force, \[\therefore \] \[{{13}^{2}}={{5}^{2}}+{{12}^{2}}+2\times 5\times 12\cos \alpha \] \[\Rightarrow \]\[169=169+120\,\cos \alpha \] \[\Rightarrow \]\[\cos \alpha =0\] \[\Rightarrow \]\[\alpha ={{90}^{o}}\]
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