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In Figure, tangents PQ and PR are drawn to a circle such that \angleRPQ = 30^{\circ}. A chord RS is drawn parallel to the tangent PQ. Find the \angleRQS.

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Solution
In the given figure PQ and PR are two tangents drawn from an external point P. 
\therefore PQ = PR      [\mathbb{Q} lengths of tangents drawn from on external point to a circle are equal]
\Rightarrow \angle PQR=\angle QRP   [  angels opposite to equal sides are equal]
  In \bigtriangleupPQR
\angle PQR+\angle QRP+\angle RPQ= 180^{\circ}
                              [  sum of angels of a triangle is 180°]
\angle PQR+\angle PQR+\angle RPQ= 180^{\circ}
\left [ \angle PQR= \angle QRP \right ]
2\angle PQR+30^{\circ}= 180^{\circ}
\angle PQR= \frac{180^{\circ}-30^{\circ}}{2}
\angle PQR= \frac{150^{\circ}}{2}
\angle PQR= 75^{\circ}

SR||OP (Given)

\therefore  \angle SRQ= \angle RQP= 75^{\circ} [Alternate interior angles]      

  Also     \angle PQR= \angle QRS= 75^{\circ}   [Alternate segment angles]    
In \bigtriangleupQRS
\angle Q+\angle R+\angle S= 180^{\circ}
\angle Q+75^{\circ}+75^{\circ}= 180^{\circ}
\angle Q= 180^{\circ}-75^{\circ}-75^{\circ}
\angle Q= 30^{\circ}
\therefore \angle RQS= 30^{\circ}

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