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ABCD quadrilateral.    angle ABC = 90 deg.
               hence,  AB² + BC² = AC²     --- (1)    using Pythagoras theorem.

AD² = ( AB² + BC² ) + CD²  given,
     =  AC² + CD²          ---- (2)

In the triangle ACD,  AD has to be a hypotenuse and angle ACD must be 90 deg. as per Pythagoras theorem.

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