Given, draw two circles having centres O and O' intersect at points A and B.
Also, draw line PQ parallel to OO'.
Construction Join OO', OP, O'Q, OM and O'N.
To prove PQ=2OO'
Proof In `DeltaOPB, BM=MP ` [OM is the perpendicular bisector of PB]
and in `DeltaO'BQ, BN=NQ` [O' N is the perpendicular bisector of BQ]
`:. BM+BN=PM+NQ`
`rArr 2(BM+BN)=BM+BN+PM+NQ`
[adding both sides BM+BN]
`rArr 2OO'=(BM+MP)+(BN+NQ)`
`[:' OO' =MN=MB+BN]`
=BP+BQ=PQ
`rArr 2OO'=PQ`