Center this cirle at the origin of a graphing plane.
If the equation of the cirlce is x2 + y2 = r2,
the endpoints of the horizonatal diameter through the origin are A = (-r,0) and B = (r,0).
Choose the point P to have coordinate of (p,0).
Choose the chord parallel to the diameter to have endpoints C =(-a,b) and D = (a,b).
Using the distance formula:
PC = sqrt( (p + a)2 + (0 - b)2 ) ---> PC2 = (p + a)2 + (0 - b)2 ---> p2 + 2ap + a2 + b2
PD = sqrt( (p - a)2 + (0 - b)2 ) ---> PD2 = (p - a)2 + (0 - b)2 ---> p2 - 2ap + a2 + b2
-----> PC2 + PD2 = ( p2 + 2ap + a2 + b2 ) + ( p2 - 2ap + a2 + b2 ) = 2p2 + 2a2 + 2b2
Choosing P to be on the radius from the origin to point B:
PA = p + r and PB = r - p
-----> PA2 + PB2 = (p + r)2 + (r - p)2 = ( p2 + 2pr + r2 ) + ( p2 - 2pr + r2 ) = 2p2 + 2r2
But, since the equation for the circle is x2 + y2 = r2 and the point (a,b) is on the circle a2 + b2 = r2
2p2 + 2a2 + 2b2 ---> 2p2 + 2r2
So, they are equal.
You can choose P to be on the radius from the origin to point A and get the same result.