Question:

The order of the differential equation of all circles passing through the origin and having their centres on the \(x\)-axis is

Show Hint

The order of the differential equation of a family of curves is usually equal to the number of arbitrary constants in the family.
  • \(4\)
  • \(3\)
  • \(2\)
  • \(1\)
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The Correct Option is D

Solution and Explanation

Concept: The order of a differential equation is equal to the highest order derivative present in it. For a family of curves, the order is generally equal to the number of arbitrary constants.

Step 1:
Since the centres are on the \(x\)-axis, let the centre be: \[ (a,0) \]

Step 2:
Since the circle passes through the origin, its radius is: \[ r=a \]

Step 3:
Equation of the circle is: \[ (x-a)^2+y^2=a^2 \] Expanding: \[ x^2-2ax+a^2+y^2=a^2 \] \[ x^2+y^2-2ax=0 \]

Step 4:
This family contains only one arbitrary constant \(a\).
So we need to differentiate once to eliminate \(a\).

Step 5:
Differentiate: \[ x^2+y^2-2ax=0 \] \[ 2x+2y\frac{dy}{dx}-2a=0 \] The highest derivative present is: \[ \frac{dy}{dx} \] So the order is: \[ 1 \] Therefore, \[ \boxed{1} \]
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