If the two sides of a pair of opposite sides of a cyclic quadrilateral are equal, prove that its diagonals are equal.

Explanation

To prove that the diagonals of a cyclic quadrilateral are equal when a pair of opposite sides are equal, we can follow these steps:

### Step-by-Step Solution:

1. **Given Information**: Let the cyclic quadrilateral be \( ABCD \) where \( AD = BC \).

2. **Understanding Arcs**: Since \( ABCD \) is a cyclic quadrilateral, the opposite angles subtend arcs on the circle. Given that \( AD = BC \), we can infer that the arcs subtended by these sides are equal. Therefore, we have:
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