MHT CET · Maths · Circle

The differential equation of family of circles, whose centres are on the X -axis and also touch the Y -axis is

A \(4\left(x+y \frac{\mathrm{~d} y}{\mathrm{~d} x}\right)^2 x^2=\left(x^2+y^2\right)^2\)
B \(\left(x+y \frac{\mathrm{~d} y}{\mathrm{~d} x}\right)^2 x^2=\left(x^2+y^2\right)^2\)
C \(2\left(x+y \frac{\mathrm{~d} y}{\mathrm{~d} x}\right)^2 x^2=\left(x^2+y^2\right)^2\)
D \(\left(x+y \frac{\mathrm{~d} y}{\mathrm{~d} x}\right)^2 x^2=4\left(x^2+y^2\right)^2\)

Verified Solution

Answer & Solution

Correct Answer

(A) \(4\left(x+y \frac{\mathrm{~d} y}{\mathrm{~d} x}\right)^2 x^2=\left(x^2+y^2\right)^2\)

Step-by-step Solution

Detailed explanation

The system of circles whose centre lies on X -axis and touch Y -axis (i.e., passes through the origin) is
\(x^2+y^2=2 \mathrm{~b} x...(i)\)
Differentiating w.r.t \(x\), we get
\(x+y \frac{\mathrm{~d} y}{\mathrm{~d} x}=\mathrm{b}...(ii)\)
Substituting (ii) in (i), we get
\(\begin{aligned}
& x^2+y^2=2\left(x+y \frac{\mathrm{~d} y}{\mathrm{~d} x}\right) x \\
& \Rightarrow\left(x^2+y^2\right)^2=4\left(x+y \frac{\mathrm{~d} y}{\mathrm{~d} x}\right)^2 x^2
\end{aligned}\)

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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