• 7 CATs FREE!
    If you earn 100 Forum Points

    Engage in the Beat The GMAT forums to earn
    100 points for $49 worth of Veritas practice GMATs FREE

    Veritas Prep
    VERITAS PRACTICE GMAT EXAMS
    Earn 10 Points Per Post
    Earn 10 Points Per Thanks
    Earn 10 Points Per Upvote
    REDEEM NOW

Area of triangle ABC

This topic has 2 expert replies and 0 member replies

Area of triangle ABC

Post
Hi GMAT Experts,

Can you please help me on why S1 is not sufficient in the below DS question.

My rational was that according to S1, ABD=60, so ADB must be 90 and we know the length of AD, so we can calculate the lengths of all the sides , SInce BAD must be 30 degrees, DAC also should be 30 degrees and ACD must be 60 degrees so with this, I can calculate DC as well.

But it looks like this is not correct. Can you help me on what am I missing.

Source : Manhattan CAT

  • +1 Upvote Post
  • Quote
  • Flag
Post
kamalakarthi wrote:
Can you please help me on why S1 is not sufficient in the below DS question.

My rational was that according to S1, ABD=60, so ADB must be 90 and we know the length of AD, so we can calculate the lengths of all the sides , SInce BAD must be 30 degrees, DAC also should be 30 degrees and ACD must be 60 degrees so with this, I can calculate DC as well.
From the given information, we know that ABD and ACD are both right triangles. However, since we don't know the angle of B or C, we don't know if they are SYMMETRICAL triangles. Don't just trust your eyes and assume that they are the same! One triangle might be slightly larger or smaller than the other.

Statement (1) tells us that ABD is a 30-60-90 triangle, and thus we can calculate its lengths and area. There is no information given to allow us to infer that DAC is also 30, as you stated. It might be slightly smaller or slightly larger. We don't have sufficient information to find the area of the entire triangle.

Statement (2) likewise only gives us information about triangle ACD, but not ABD. If AC = 12, we can infer that ACD is a 30-60-90, but this won't tell us the area of the whole triangle ABC.

(1) & (2) together: if ABD and ACD are both 30-60-90 triangles with a shared side, they must be identical. Since we know that they are both 30-60-90 triangles and we have a side length for each, we have sufficient information to determine the area of each.

The answer is C.

_________________


Ceilidh Erickson
Manhattan Prep GMAT & GRE instructor
EdM in Mind, Brain, and Education
Harvard Graduate School of Education


Manhattan Prep instructors all have 99th+ percentile scores and expert teaching experience.
Sign up for a FREE TRIAL, and learn why we have the highest ratings in the GMAT industry!

  • +1 Upvote Post
  • Quote
  • Flag
Free Manhattan Prep online events - The first class of every online Manhattan Prep course is free. Classes start every week.
Post
Trusting your eyes & making assumptions are two of the biggest mistakes you can make on DS geometry problems! Here's an article about why geometry is especially tricky in DS: https://www.manhattanprep.com/gmat/blog/2016/05/13/heres-why-youre-getting-geometry-problems-on-gmat-data-sufficiency-wrong/

_________________


Ceilidh Erickson
Manhattan Prep GMAT & GRE instructor
EdM in Mind, Brain, and Education
Harvard Graduate School of Education


Manhattan Prep instructors all have 99th+ percentile scores and expert teaching experience.
Sign up for a FREE TRIAL, and learn why we have the highest ratings in the GMAT industry!

  • +1 Upvote Post
  • Quote
  • Flag
Free Manhattan Prep online events - The first class of every online Manhattan Prep course is free. Classes start every week.
  • Economist Test Prep
    Free Trial & Practice Exam
    BEAT THE GMAT EXCLUSIVE

    Available with Beat the GMAT members only code

    MORE DETAILS
    Economist Test Prep
  • PrepScholar GMAT
    5 Day FREE Trial
    Study Smarter, Not Harder

    Available with Beat the GMAT members only code

    MORE DETAILS
    PrepScholar GMAT
  • e-gmat Exclusive Offer
    Get 300+ Practice Questions
    25 Video lessons and 6 Webinars for FREE

    Available with Beat the GMAT members only code

    MORE DETAILS
    e-gmat Exclusive Offer
  • Kaplan Test Prep
    Free Practice Test & Review
    How would you score if you took the GMAT

    Available with Beat the GMAT members only code

    MORE DETAILS
    Kaplan Test Prep
  • The Princeton Review
    FREE GMAT Exam
    Know how you'd score today for $0

    Available with Beat the GMAT members only code

    MORE DETAILS
    The Princeton Review
  • Varsity Tutors
    Award-winning private GMAT tutoring
    Register now and save up to $200

    Available with Beat the GMAT members only code

    MORE DETAILS
    Varsity Tutors
  • Veritas Prep
    Free Veritas GMAT Class
    Experience Lesson 1 Live Free

    Available with Beat the GMAT members only code

    MORE DETAILS
    Veritas Prep
  • EMPOWERgmat Slider
    1 Hour Free
    BEAT THE GMAT EXCLUSIVE

    Available with Beat the GMAT members only code

    MORE DETAILS
    EMPOWERgmat Slider
  • Magoosh
    Magoosh
    Study with Magoosh GMAT prep

    Available with Beat the GMAT members only code

    MORE DETAILS
    Magoosh
  • Target Test Prep
    5-Day Free Trial
    5-day free, full-access trial TTP Quant

    Available with Beat the GMAT members only code

    MORE DETAILS
    Target Test Prep

Top First Responders*

1 Brent@GMATPrepNow 68 first replies
2 fskilnik@GMATH 56 first replies
3 Jay@ManhattanReview 39 first replies
4 GMATGuruNY 35 first replies
5 Rich.C@EMPOWERgma... 31 first replies
* Only counts replies to topics started in last 30 days
See More Top Beat The GMAT Members

Most Active Experts

1 image description fskilnik@GMATH

GMATH Teacher

130 posts
2 image description Brent@GMATPrepNow

GMAT Prep Now Teacher

101 posts
3 image description Max@Math Revolution

Math Revolution

84 posts
4 image description Rich.C@EMPOWERgma...

EMPOWERgmat

79 posts
5 image description Jay@ManhattanReview

Manhattan Review

76 posts
See More Top Beat The GMAT Experts