Bunuel wrote:

eybrj2 wrote:

Which of the figures below can be inscribed in a circle?

A. 1 only

B. 3 only

C. 1 & 3 only

D. 2 & 3 only

E. 1, 2 & 3

Useful property:

a convex quadrilateral can be inscribed in a circle if and only its opposite angles are supplementary (supplementary angles are two angles that add up to 180°, whereas complementary angles are two angles that add up to 90°).

We can see on the diagram that only I and III meet that requirements.

Answer: C.

Hope it's clear.

I agree with "the opposite angles are supplementary" concept and I did the same.

Can this be a solution @Bunuel ?

1. for first diagram - Moreover, isn't the first one isoceles trapezium ? Isn't that sufficient ?

2. for third diagram - Parallelogram inscribed in a circle will become a rectangle (think of shape shifting)