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10-1.Circle and System of Circles
hard
रेखा $x = y$ एक वृत्त को बिन्दु $(1,1)$ पर स्पर्श करती है। यदि यह वृत्त बिन्दु $(1,-3)$ से भी होकर जाता है, तो इसकी त्रिज्या है
A
$3\sqrt 2$
B
$3$
C
$2$
D
$2\sqrt 2$
(JEE MAIN-2019)
Solution

Equation of circle is given as $S + \lambda L = 0$
${\left( {x – 1} \right)^2} + {\left( {y – 1} \right)^2} + \lambda \left( {x – y} \right) = 0$
through $\left( {1, – 3} \right)$
$16 + \lambda \times 4 = 0 \Rightarrow \lambda = – 4$
$\therefore {\left( {x – 1} \right)^2} + {\left( {y – 1} \right)^2} + 4\left( {x – y} \right) = 0$
$r = 2\sqrt 2 $
Standard 11
Mathematics