11. Prove that the parallelogram circumscribing a circle is a rhombus.
To prove - the parallelogram circumscribing a circle is a rhombus
Proof-
ABCD is a parallelogram which circumscribes a circle with centre O.
P, Q, R, S are the points of contacts on sides AB, BC, CD and DA respectively
AB = CD .and AD = BC...........(i)
It is known that, tangents drawn from an external points are equal in length.
RD = DS ...........(ii)
RC = QC...........(iii)
BP = BQ...........(iv)
AP = AS .............(v)
By adding eq (ii) to eq (v) we get;
(RD + RC) + (BP + AP) = (DS + AS) + (BQ + QC)
CD + AB = AD + BC
2AB = 2AD [from equation (i)]
AB = AD
Now, AB = AD and AB = CD
AB = AD = CD = BC
Hence ABCD is a rhombus.