Q.From an external point \(P\), two tangents \(PS\) and \(PT\) are drawn to a circle with center \(O\). \(QS\) is a chord of the circle that is parallel to \(PT\). If \(\angle SPT = 80^\circ\), then what is the measure of \(\angle QST\)?

In triangle PST, PS = PT \[ \therefore \angle PST = \angle PTS = \cfrac{180^\circ - 80^\circ}{2} = 50^\circ \] Again, since QS \(\parallel\) PT \[ \therefore \angle PSQ = 180^\circ - 80^\circ = 100^\circ \] Therefore, \[ \angle QST = \angle PSQ - \angle PST = 100^\circ - 50^\circ = 50^\circ \]
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