Question P6 Angle chasing
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A framer is building a triangular display easel EAD, braced by a parallelogram strut BCDE, where B lies on strut AD. The two strut pieces AB and BE are cut to equal length. Given angle DBC = 42° and angle BCD = 66°, find angle EAB.
Answer
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One angle gives you the next, until you reach the angle asked for.
In this one. Once the parallelogram hands you the angle on one side of B, the straight line AD turns it into the exterior angle of the isosceles triangle — and an exterior angle is the two equal base angles added, so the answer is simply half of it.
A figure has several angles marked with only one or two values given, and words like 'isosceles', 'parallel', or 'straight line' point to which rule to use first.
Children apply an angle rule to the wrong triangle or the wrong straight line in a crowded figure, or stop one step before the angle actually asked for.
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