Q. Find unknown sizes of the angles.

Soln: 
<PQR = <QRS [∵ PQ || RS, being alternate angles]
 or, y = 2x

From r PQR,
<PQO + <OPQ = <POR
[∵ an exterior angle of a triangle is equal to the sum of the opposite interior angles.]
[Some books prefer to write: External property of a triangle]
x + y = 870
x + 2x = 870
x = 290

y = 2x = 2 * 290 = 580

and 2x = 2 * 290 = 580


Hence, <SPQ = 290, <PQR = 580 and <SRQ = 580


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