https://www.gregmat.com/problems/quant?problem=a-circle-with-radius-5-contains-an-inscribed
When can I assume diagrams are to scale? To my understanding if I don’t assume it is to scale here the answer would be D, but sometimes I cannot assume diagrams are to scale. Is there a rule for when I should and when I shouldn’t?
Generally, Query about Question 7 from "GRE Mini Exam #2" [1229] - #2 by Kweku is how ETS tests diagrams.
Generally, don’t use apparent measurements from a diagram unless the question gives you mathematical information that establishes them.
In this specific question, we don’t need to assume the diagram is to scale. The fact that the polygons are regular and inscribed forces exact central angles.
The equilateral triangle divides the circle into 120° arcs, while the regular pentagon divides it into 72° arcs. Therefore, the positions of A and B relative to their shared top vertex are mathematically determined, not estimated from the picture.
Thanks! What I mean is when can I assume that the top vertex is indeed shared without additional information such as “both have a vertex at point A” or some such.
And according to the quant reasoning description you posted this question should not be assumed to be to scale 
that’s why I’m confused as usually I wouldn’t assume it to be to scale, making the problem unsolvable.
If the two points are on the same point, you can assume they share that point.
You can trust relative positions, don’t trust measurement or scale.
Read after it says “…not necessarily to scale”. It says "You should assume, however, that lines shown as straight are actually straight, points on a line are in the order shown, and more generally, all geometric objects are in relative positions shown. For questions with geometric figures, you should base your answers on geometric reasoning, not on estimating or comparing quantities by sight or by measurement.
This is how we know the top vertex is shared.
ahh I see I see thanks! that clears it up, sorry I just took a cursory glance at it. Appreciate it a lot!
I had a similar question here. I was kind of rotating the pentagon, because ideally, if it just says regular triangle and regular pentagon, they can start and end anywhere right? They don’t always have to share the vertex unless otherwise mentioned? Because the QC diagrams alone can’t be trusted to draw conclusions.
Yes to both questions.
Yes.