In triangle ABC, if AB=
a, BC=
b+1/2, and AC=
b, which of the three angles has the
greatest degree measure?
(1)
b=
a+3/2
(2)
a=1/2
Question from GMATFree dot com - Data Sufficiency, Module 37 #7. Their explanation is also on Youtube at watch?v=h_owg_Q_25Y
This question is similar to the
greatest degree measure question from
the Official Guide (Question 19 page 276, that question is discussed on the forum here. I can't post URLs yet, but if you search "Greatest Degree Measure" it comes up.) but it uses different numbers, & statements (1) and (2) are slightly different.
The answer from GMATFree is A. (1) alone is sufficient but (2) alone is not sufficient.
However, I think (2) alone would also be sufficient & therefore the answer would be D.
If
a=1/2, then no matter what value
b is, side BC will always be longest & therefore angleBAC will always be the greatest.
- Example 1: With
a=1/2, if we say that
b=5, then side AB is the smallest at 1/2 & side BC is the biggest at
b+1/2 or 5+1/2=5.5.
- Example 2: If we say that
b=0.001, then side AC is the smallest at 0.001 & side BC is still the biggest at
b+1/2 or 0.001+1/2=0.501.
With the information
a=1/2, we don't always know for sure which side is smallest, but we always know which side is biggest (side BC), which tells us which angle is always biggest (angleBAC), which is what the question is asking.
My guess is that the GMATFree website used the explanation of the Official Question from
the Official Guide (that I noted above) as their answer for this question, but it is incorrect because the numbers used and the statements are different. Can anyone confirm this, or am I missing something? Thanks!
From the question, we already know that angle A is greater than angle B, because \(b+\frac{1}{2} > b\).
So our question simply ask the relationship between angle A and angle C, or the relationship between \(b+\frac{1}{2}\) and \(a\).
--> \(b+\frac{1}{2} ... a\)
- We subsitute b in our question, become ; \((a+\frac{3}{2})+\frac{1}{2} ... a\) --> \(a+2 ... a\), clearly \(a+2>2\), so the biggest angle is angle A.
- Substitute again --> \(b+\frac{1}{2} ... \frac{1}{2}\) --> \(b+\frac{1}{2}>\frac{1}{2}\)
- Again, we know that angle A is the biggest.