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In triangle $PQR,$ $M$ is the midpoint of $\overline{QR}.$ Find $PM.$
PQ = 5, PR = 8, QR = 11

 Apr 8, 2024
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Using Law of Cosine in \(\triangle PQR\) gives

 

\(8^2 = 5^2 + 11^2 - 2(5)(11) \cos \angle Q\\ \cos \angle Q = \dfrac{5^2 + 11^2 - 8^2}{2(5)(11)}\\ \cos \angle Q = \dfrac{41}{55}\)

 

Using Law of Cosine in \(\triangle PQM\) gives

 

\(PM^2 = 5^2 + \left(\dfrac{11}2\right)^2 - 2(5)\left(\dfrac{11}2\right) \cos \angle Q\\ PM^2 = 5^2 + \left(\dfrac{11}2\right)^2 - 2(5)\left(\dfrac{11}2\right)\left(\dfrac{41}{55}\right)\\ PM^2 = \dfrac{57}4\\ PM = \dfrac{\sqrt{57}}2\)

 Apr 8, 2024

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