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chigpt
How ABD = AED??


Sent from my iPhone using GMAT Club Forum

That must be a typo in @BeingHan's solution because Angle ABD = angle ACD. Check here: https://gmatclub.com/forum/in-the-figur ... l#p1893868
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In the figure shown below AB = AC, angle BAD = 30°, and AE = AD. Then x equals:


(A) 7.5°

(B) 10°

(C) 12.5°

(D) 15°

(E) 20°

Attachment:
shot21.jpg

This question is testing exterior angles and related isosceles triangles. For the triangle (ABD) on the left, we can choose an angle for the base of the bigger triangle i.e 50, therefore the external angle would be 80 (30+50) and this would include x. Since we chose 50 for the base angles the third angle of the whole triangle (ABC) would be 180-100= 80, then the third angle of the triangle on the right(ADE) would be 50 (80-30) and the base angles would be 130/2= 65 each. The left base angle of ADE plus x equals 80, then x =80 -65 = 15. Answer is D
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carcass
In the figure shown below AB = AC, angle BAD = 30°, and AE = AD. Then x equals:


(A) 7.5°

(B) 10°

(C) 12.5°

(D) 15°

(E) 20°

Attachment:
shot21.jpg

Ans (D) 15
Let angle ABD = Z, since AB=AC, then angle ACB = Z
In Triangle BAC, angle BAC = 180-2z, hence angle DAE = (180-2z) - 30 = 150-2z --------(1)

Now in smaller triangle ADE, let angle ADE = y, since AD=AE, so angle AED = y
y+y+angle DAE = 180
or 2y + (150 -2z) [from (1) ] = 180
or y-z = 15 ------------(2)

Final Step, angle AED (Exterior angle) = Angle EDC + angle ECD (Sum of interior angles)
i.e. y = x + z
or y-z = x = 15 (from (2) )
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Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

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