Assume we have a circle
This property shows that if a triangle is drawn inside a semicircle, the angle opposite the diameter will be 90º.
With this property in mind, let us draw a 90º angle at a point on the circumference of the circle.
We can repeat the process again, and draw another right angle. By extending the lines formed by the right angle, another triangle inscribed in the semicircle is formed. Again, the 3rd side of this triangle is the diameter.
2) How to find the radius of a circle
Assuming we have another circle
Draw another line down from the point where the lines meet. Since the lines are of equal length, the line drawn here will bisect the diameter, forming 2 radii. The line that is drawn down to meet the diameter is also another radius.
If the center has already been found, using methods such as in the previous part,
Draw a line from the center to the circumference, on one side. This line is the radius.
This method can be used for any other circle which the center is already known.
Otherwise, calculation can be used. If the diameter, circumference or area of the circle is known, their respective formulas can be applied to obtain the radius.
Measurement can also be used.