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Q.22. If $\overrightarrow{a} and \overrightarrow{b}$ are two unit vectors such that $\overrightarrow{a}+\overrightarrow{b}$ is also a unit vector, then show that the angle between $\overrightarrow{a} and \overrightarrow{b}$ is $\frac{2\mathrm{\pi }}{3}$.