Bunuel wrote:
In the diagram, JKLM is a square, and P is the midpoint of KL. Is JQM an equilateral triangle?
(1) Angle KPQ = 90°
(2) Angle JQP = 150°
Kudos for a correct solution. MAGOOSH OFFICIAL SOLUTION:First of all, clearly if we knew that that JQM is equilateral, then we could know that ÐKPQ = 90° and that ÐJQP = 150°. That much is clear. An equilateral triangle is consistent with the statements, but the question is, does either statement or do both of them necessitate that JQM be equilateral?
Statement #1: Angle KPQ = 90°
This statement guarantees that Q is directly below P, so that PQ lies on the vertical midline of the square. This guarantees that JQM is isosceles, but Q could be at any height.
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This statement guarantees that JQM is isosceles. It may or may not be equilateral. This statement, alone and by itself, is not sufficient.
Statement #2: Angle JQP = 150°
Now, that obtuse angle is fixed, but we don’t know whether Q is still on that vertical midline. This could be the case in which JQM is equilateral, or it could be entirely asymmetrical.
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With the restriction of this statement, it could be true that triangle JQM is equilateral, but as in these diagram, JQM might be completely asymmetrical. We can’t give a definitive answer. This statement, alone and by itself, is not sufficient.
Combined statements: Angle KPQ = 90° & angle JQP = 150°
The first statement tells us that PQ is on the midline, and that JQM must be isosceles. The entire diagram is symmetrical over this vertical midline. This means, if ÐJQP = 150°, then ÐMQP = 150° as well, and because the angles around point Q must add up to 360°, this means that ÐJQM must equal 60°. Well, any isosceles with one 60° angle must be equilateral. We absolute know that triangle JQM must be isosceles now. This is definitive. The statements together are sufficient.
Answer = (C)