Consider a circle with center O, and draw a diameter AB
Now, we have to prove that the intersectio point of lines drawn from the two diameteric ends meeting at the circumference subtends a right angle.
Suppose they meet on the circumference at point P
Diameter will subtend an angle of 180 degrees at the center.
The angle subtended at the center is equal to half of the angle subtended at any point on the circumference hence the angle subtended at P = 1/2 × 180 = 90
Hence, proved