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
A circle meeting the axes at exactly three points must pass through the origin (one common point).
Equal intercepts and centre in the first quadrant ⇒ centre (g,g), g>0:
x2+y2-2gx-2gy=0, intercepts 2g on both axes, r2=2g2
Distance of the centre from x+y-1=0: d= |2g-1| 2
Chord length =2 r2-d2= 14
r2-d2= 72
2g2- 4g2-4g+12= 72
4g-1=7 g=2
∴ r2=2g2=8
Answer: 8