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Prove that the parallelogram circumscribing a circle is a rhombus.
 
Proof:
 
We know that, the lengths of tangents drawn from an external point to a circle are equal.
 
\(AP = \)  ----(1)
 
\(BP = \)  ----(2)
 
\(CR = \)  -----(3)
 
\(DR = \)  -----(4)
 
By, adding (1), (2), (3) and (4) RHS = LHS, we get
 
\(AP + BP + CR + DR = AS + BQ + CQ + DS (AP + BP) + (CR + DR)\)
 
\(= (AS + DS) + (BQ + CQ)\) 
 
\( + CD = AD +\)
 
As \(ABCD\) is a, \(AB = CD\) and \(AD = BC\)
 
Hence, \(2 AB = 2 BC\)
 
\(AB = BC\)
 
If the of a parallelogram are equal, then it is a rhombus.
 
Hence, \(ABCD\) is a rhombus.