Question:

Segment \( PM \) is a median of \( \triangle PQR \), \( PM = 9 \), and \( PQ^2 + PR^2 = 290 \). Find \( QR \).

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The Apollonius theorem is useful for finding side lengths in triangles when medians are involved.
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Solution and Explanation

Step 1: Using the Apollonius theorem for medians in a triangle: \[ PQ^2 + PR^2 = 2PM^2 + \frac{1}{2}QR^2. \] Step 2: Substituting the given values \( PQ^2 + PR^2 = 290 \), \( PM = 9 \): \[ 290 = 2(9^2) + \frac{1}{2}QR^2. \] Step 3: Simplify: \[ 290 = 2(81) + \frac{1}{2}QR^2. \] \[ 290 = 162 + \frac{1}{2}QR^2. \] Step 4: Rearrange and solve for \( QR^2 \): \[ 290 - 162 = \frac{1}{2}QR^2. \] \[ 128 = \frac{1}{2}QR^2 \quad \Rightarrow \quad QR^2 = 256. \] Step 5: Taking the square root: \[ QR = \sqrt{256} = 16. \]
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