Two forces P,Q act at a point along two straight lines making an angle \(\alpha\) with each other and R is their resultant . Two other forces P', Q' acr along the same two lines and have a resultant R'. prove that the angle between the line of acting the resultant is
\(\cos^{-1} \)(PP'+QQ'+(PQ'+P'Q)cos\(\alpha\))/RR'.
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