JEE Advanced · Mathematics · 12. Circle
The circle passing through the point \((-1,0)\) and touching the \(Y\)-axis at \((0,2)\), also passes through the point
- A
\(\left(-\frac{3}{2}, 0\right)\)
- B
\(\left(-\frac{5}{2}, 2\right)\)
- C
\(\left(-\frac{3}{2}, \frac{5}{2}\right)\)
- D
\((-1,-4)\)
Answer & Solution
Correct Answer
(D)
\((-1,-4)\)
Step-by-step Solution
Detailed explanation
Equation of circle passing through a point \(\left(x_1, y_1\right)\) and touching the straight line \(L\), is given by
\[
\left(x-x_1\right)^2+\left(y-y_1\right)^2=\lambda L=0
\]
\(\therefore\) Equation of circle passing through \((0,2)\) and touching \(x=0\).
Now, \((x-0)^2+(y-2)^2+\lambda x=0 \ldots\) (i)
Also, it passes through \((-1,0)\).
So, \(1+4-\lambda=0 \Rightarrow \lambda=5\)
Eq. (i) becomes,
\[
\begin{aligned}
& x^2+y^2-4 y+4+5 x=0 \\
\Rightarrow \quad x^2+y^2+5 x-4 y+4 & =0
\end{aligned}
\]
For \(x\)-intercept, put \(y=0\),
\[
\begin{array}{rlrl}
& x^2+5 x+4 & =0 \\
\Rightarrow & & (x+1)(x+4) & =0 \\
\therefore & & x & =-1,-4
\end{array}
\]
\[
\left(x-x_1\right)^2+\left(y-y_1\right)^2=\lambda L=0
\]
\(\therefore\) Equation of circle passing through \((0,2)\) and touching \(x=0\).
Now, \((x-0)^2+(y-2)^2+\lambda x=0 \ldots\) (i)
Also, it passes through \((-1,0)\).
So, \(1+4-\lambda=0 \Rightarrow \lambda=5\)
Eq. (i) becomes,
\[
\begin{aligned}
& x^2+y^2-4 y+4+5 x=0 \\
\Rightarrow \quad x^2+y^2+5 x-4 y+4 & =0
\end{aligned}
\]
For \(x\)-intercept, put \(y=0\),
\[
\begin{array}{rlrl}
& x^2+5 x+4 & =0 \\
\Rightarrow & & (x+1)(x+4) & =0 \\
\therefore & & x & =-1,-4
\end{array}
\]
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