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Question -

In a тИЖABC, if AB = AC and BC is produced to D such that тИаACD = 100┬░, then тИаA =
(a) 20┬░
(b) 40┬░
(c) 60┬░
(d) 80┬░



Answer -

In тИЖABC, AB = AC
тИ┤ тИаB = тИаC
But Ext. тИаACD = тИаA + тИаB
тИаACB + тИаACD = 180┬░ (Linear pair)
тИ┤ тИаACB + 100┬░ = 180┬░
тЗТ тИаACB = 180┬░-100┬░ = 80┬░
тИ┤ тИаB = тИаACD = 80┬░
But тИаA + тИаB 4- тИаC = 180┬░
тИ┤ тИаA + 80┬░ + 80┬░ = 180┬░
тЗТтИаA+ 160┬░= 180┬░
тИ┤ тИаA= 180┬░- 160┬░ = 20┬░ (a)

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