The line x=y touches a circle at the point (1,1). If the circle also passes through the point (1,−3), then its radius is
(A)32
(B)3
(C)2
(D)22✓ Correct
Answer: (D)
Solution
A circle touching the line x=y at (1,1) can be written (x−1)2+(y−1)2+λ(x−y)=0. Forcing it through (1,−3) gives 0+16+λ(1+3)=0, so λ=−4. The equation becomes (x−1)2+(y−1)2−4(x−y)=0, i.e. x2+y2−6x+2y+2=0, whose radius is 9+1−2=8=22.
JEE Main · 2019 · 10 Apr 2019 · Shift 1 · Single correct option