Geometry question that has me kinda perplexed
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- Senior | Next Rank: 100 Posts
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- Joined: Fri Jun 01, 2007 11:02 pm
Got this one wrong myself. Here's what I learned:
Angle BDC = 2x is an exterior angle of triangle ABD.
Hence, it is the sum of the two non-adjacent angles. This is the key rule I didn't know beforehand. You can plug in numbers to find out that this will always be true.
That is: angle BDC = angle BAD + angle ABD
Hence: 2x = x + angle ABD
Then: angle ABD = x
Once you know this, you know AD = BD because it's an isosceles triangle, and likewise for BD = BC. Thus, AD = BD = BC.
So (1) is sufficient.
Angle BDC = 2x is an exterior angle of triangle ABD.
Hence, it is the sum of the two non-adjacent angles. This is the key rule I didn't know beforehand. You can plug in numbers to find out that this will always be true.
That is: angle BDC = angle BAD + angle ABD
Hence: 2x = x + angle ABD
Then: angle ABD = x
Once you know this, you know AD = BD because it's an isosceles triangle, and likewise for BD = BC. Thus, AD = BD = BC.
So (1) is sufficient.