Hi GreenDay14,

Locate the circle centre and draw lines from it to all 4 corners of the quadrilateral.

Since each resulting triangle has 2 sides of radius length, they are 4 isosceles triangles.

Now label equal angles at the circumference,

for example A, A, B, B, C, C, D, D.

Then (2A+2B)+(2C+2D)=360 degrees.

(A+B)+(C+D)=180 degrees.

Also (A+D)+(B+C)=180 degrees.