The question seems to be wrong. The question should have been "Prove that d22=d12+C2
AB=c2 [As c→ tangent to the inner circle
OA⊥AB at A and it divides the chord of length C into 2 equal half]
In ΔOAB,
By Pythagoras theorem,
OB2=OA2+AB2
(d22)2=(d12)2+(c2)2
∴d22=d21+c2.