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chetan2u
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chetan2u

Is triangle ABC a right-angled \(\triangle\), if BD=DC and \(\angle ACB =60\)?


(1) AB is parallel to DE.

(2) D is the midpoint of AC.


Chetan's questions

Attachment:
Untitled01.png

Typically GMAT would word the question as "If BD=DC and \(\angle ACB =60\), is triangle ABC a right-angled triangle?"

From the graph we are given \(\triangle DEC\) is a right triangle with \(\angle C = 60\).

Statement 1:

We have \(\angle DEC = \angle ABC = 90\) so triangle ABC a right triangle. Sufficient.

Statement 2:

BD = DC = AD, so BD is half of AC. From Thales theorem it follows that ABC is a right triangle. Sufficient.

Ans: D
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chetan2u

Beautiful solution. What I didn't know was the following for statement 2:

BD=DC and BD=DA

This allowed you to determine that you are looking at two isosceles. Can you elaborate more on how you knew this? That's why I wasn't able to solve because I didn't see that...
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chetan2u

Is triangle ABC a right-angled \(\triangle\), if BD=DC and \(\angle ACB =60\)?


(1) AB is parallel to DE.

(2) D is the midpoint of AC.


Chetan's questions

Attachment:
Untitled01.png

Typically GMAT would word the question as "If BD=DC and \(\angle ACB =60\), is triangle ABC a right-angled triangle?"

From the graph we are given \(\triangle DEC\) is a right triangle with \(\angle C = 60\).

Statement 1:

We have \(\angle DEC = \angle ABC = 90\) so triangle ABC a right triangle. Sufficient.

Statement 2:

BD = DC = AD, so BD is half of AC. From Thales theorem it follows that ABC is a right triangle. Sufficient.

Ans: D

Why does it necessarily follow that BD = DC = AD?
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