**ABC and ADC are two right triangles with common hypotenuse AC. Prove that .**

**Solution:**

AC is the common hypotenuse for two right triangles, and .

** (Given)**

**(If sum of a pair of opposite angles of a quadrilateral is , the quadrilateral is cyclic.)**

**Therefore, quadrilateral ABCD is cyclic.**

. ** (Angles in the same segment are equal)**