In a quadrilateral ABCD, `angleA+angleD=90^(@)`. Prove that
`AC^(2)+BD^(2)=AD^(2)+BC^(2)`

Explanation

To prove that in quadrilateral ABCD, if \( \angle A + \angle D = 90^\circ \), then \( AC^2 + BD^2 = AD^2 + BC^2 \), we will follow these steps:

### Step 1: Understand the Given Information
We are given a quadrilateral ABCD where \( \angle A + \angle D = 90^\circ \).

### Step 2: Extend the Lines
Extend lines AB and CD to meet at point E. This allows us to analyze triangles AED and BEC more easily.

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