Let a circle C have its centre in the first quadrant, intersect the coordinate axes at exactly three points and cut off equal intercepts from the coordinate axes. If the length of the chord of C on the line x+y=1 is 14, then the square of the radius of C is ______.
Answer: 8
Solution
Equal intercepts on the axes together with exactly three points of intersection mean the circle passes through the origin with centre on y=x, say at (p,p) with p>0; its radius is then p2+p2=p2, so r2=2p2. The distance from (p,p) to the line x+y=1 is 2∣2p−1∣. Using (2chord)2+d2=r2 with chord 14, 414+2(2p−1)2=2p2, which simplifies to 7+4p2−4p+1=4p2, i.e. 8−4p=0 and p=2. Hence the square of the radius is r2=2p2=8.