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KCET · Maths · Differential Equations

The family of curves whose \(x\) and \(y\) intercepts of a tangent at any point are respectively double the \(x\) and \(y\) coordinates of that point is

  1. A \(x y=C\)
  2. B \(x^2+y^2=C\)
  3. C \(x^2-y^2=C\)
  4. D \(\frac{y}{x}=C\)
Verified Solution

Answer & Solution

Correct Answer

(A) \(x y=C\)

Step-by-step Solution

Detailed explanation

Let the point on the curve be \((h, k)\).
According to the question, the equation of the tangent will be
\(\Rightarrow \quad \frac{x}{2 h}+\frac{y}{2 k}=1\)
\(\Rightarrow \quad \frac{y}{2 k}=1-\frac{y}{2 h}\)
\(\Rightarrow \quad y=-\left(\frac{2 k}{2 h}\right) x+2 k\)
So, the slope of tangent will be \(\left(-\frac{k}{h}\right)\).
Now, \(\quad \frac{d y}{d x}=-\frac{h}{k}=-\frac{y}{x}\)
\(\Rightarrow \quad \frac{d y}{y}=-\frac{d x}{x}\)
On integrating both sides, we get
\(\int \frac{d y}{d y}=-\int \frac{d x}{x}\)
\(\Rightarrow \quad \ln y=-\ln x+\ln C\)
\(\Rightarrow \quad \ln y=\ln \frac{C}{x}\)
\(\Rightarrow \quad y=\frac{C}{x}\)
\(\therefore \quad x y=C\)