Geometry Triangles

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vishalwin: Master | Next Rank: 500 Posts; Posts: 274; Joined: Fri Sep 18, 2015 10:58 pm; Thanked: 12 times; Followed by:1 members; GMAT Score:530

Geometry Triangles

by vishalwin » Wed Nov 25, 2015 7:40 am

Hi

Can anyone please explain the attached question.

What id the position of point D is fixed, then is it able to find answer uaing statement 1?

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: Geometry

Thanks & Regards
vishalwin
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GMAT Score - 530
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Source: Beat The GMAT — Data Sufficiency | Wed Nov 25, 2015 7:40 am

GMAT/MBA Expert

Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pm; Location: Vancouver, BC; Thanked: 5254 times; Followed by:1268 members; GMAT Score:770

by Brent@GMATPrepNow » Wed Nov 25, 2015 7:42 am

OG13 DS 149

In the figure above, what is the value of x + y
1.x = 70
2.âˆ†ABC and âˆ†ADC are both isosceles triangles

Important point: For geometry DS questions, we are typically checking to see whether the statements "lock" a particular angle or length into having just one value.

This concept is discussed in much greater detail in our free video: https://www.gmatprepnow.com/module/gmat- ... cy?id=1103

Statement 1 locks angle x by forcing it to have just one value (however it does not lock angle y).

Statement 2 forces the two triangles to be isosceles but the statement does not force the angles to have any particular values.

When we combine the statements, we should try to mentally grab points and try to move them (and still conform to the rules dictated by the statements). In this question, we can "grab" point D and pull it up or down, while still maintaining a 70-degree angle for x AND keeping the triangles as isosceles triangles. As we move point D up and down, angle y changes. So, there's now way that x+y can have a fixed value.

This means the answer is E

Cheers,
Brent

Brent Hanneson - Creator of GMATPrepNow.com

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GMAT/MBA Expert

Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pm; Location: Vancouver, BC; Thanked: 5254 times; Followed by:1268 members; GMAT Score:770

by Brent@GMATPrepNow » Wed Nov 25, 2015 7:42 am

double post

Brent Hanneson - Creator of GMATPrepNow.com

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Matt@VeritasPrep: GMAT Instructor; Posts: 2630; Joined: Wed Sep 12, 2012 3:32 pm; Location: East Bay all the way; Thanked: 625 times; Followed by:119 members; GMAT Score:780

by Matt@VeritasPrep » Fri Nov 27, 2015 1:10 am

Well, it "is" fixed, we just don't know where. From S1, we also don't know that C and D both lie on the altitude from C to AB, or that both âˆ†s are isosceles, among other things.