Bunuel wrote:
The Official Guide For GMAT® Quantitative Review, 2ND EditionIn triangle ABC above, what is the length of side BC?
(1) Line segment AD has length 6.
(2) x = 36
Target question: What is the length of side BC? Statement 1: Line segment AD has length 6. BEFORE we deal with statement 1, let's see what information we can add to the diagram.
For example, since ∆BDC has 2 equal angles (of 2x°), we know that side BD = side BC:
Next, since angles on a line add to 180°, and since ∠BDC = 2x°, we know that ∠ADB = (180 - 2x)°
Now focus on ∆BAD
Since angles in a triangle add to 180°, we know that ∠ABD = x°
ASIDE: Notice that x° + x° + (180 - 2x)° = 180°
Now that we know ∆BAD has two equal angles (x° and x°), we know that side AD = side BD
This means AD = BD = BCStatement 1 tells us that AD = 6, which means
BC = 6The answer to the target question is
side BC has length 6Since we can answer the
target question with certainty, statement 1 is SUFFICIENT
Statement 2: x = 36Notice that our diagram doesn't any lengths.
We can SHRINK or ENLARGE the diagram and the angles remain the same.
However
the length of side BC changes.
Since we cannot answer the
target question with certainty, statement 2 is NOT SUFFICIENT
Answer: A
Cheers,
Brent
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Brent Hanneson – Creator of gmatprepnow.com
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