Name: SAT Math Help & School Math Mastery
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Let $A BC D$ be a trapezoid with $A B ∥ C D$ , $A B = 13$ , $BC = 8$ , $C D = 21$ , and $D A = 10$ . Bisectors of $∠ A$ and $∠ D$ meet at $P$ , and bisectors of $∠ B$ and $∠ C$ meet at $Q$ . What is the length of segment $PQ$ ?