\textbf{156-} Three forces $\vec{F}_1$, $\vec{F}_2$, $\vec{F}_3$ make pairwise angles of $120^\circ$ with each other. If the magnitudes of the forces are 5, 10, and 15 newtons respectively, find their resultant. How many newtons is it?
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(1) zero \hfill (2) $5$ \hfill (3) $5\sqrt{3}$ \hfill (4) $10$
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