In ΔABC , if angleA - angleB=42 and angleB - angleC = 21 , then angleB = ?

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Solution

Given,
A-B=42....(1)
B-C=21.....(2)
we know that sum of interior angles of a triangle=180
so, A+B+C=180.....(3)
(1)+(2)
A-C=63....(4)
(3)-(4)
B+2C=117.....(5)
B-C=21...(2)
(2)×2
2B-2C=42..(6)
(5)+(6)
3B=159
B=53

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