Maths-
General
Easy

Question

The differential equation of all circles which pass through the origin and whose centre lies on y-axis is

  1. open parentheses x to the power of 2 end exponent minus y to the power of 2 end exponent close parentheses fraction numerator d y over denominator d x end fraction minus 2 x y equals 0    
  2. open parentheses x to the power of 2 end exponent minus y to the power of 2 end exponent close parentheses fraction numerator d y over denominator d x end fraction plus 2 x y equals 0    
  3. open parentheses x to the power of 2 end exponent minus y to the power of 2 end exponent close parentheses fraction numerator d y over denominator d x end fraction minus x y equals 0    
  4. open parentheses x to the power of 2 end exponent minus y to the power of 2 end exponent close parentheses fraction numerator d y over denominator d x end fraction plus x y equals 0    

hintHint:

We have to find the differential equation of circle who has center at y-axis and is touching the origin. We will first write the equation of the circle. Then we will differentiate and find the value.

The correct answer is: open parentheses x to the power of 2 end exponent minus y to the power of 2 end exponent close parentheses fraction numerator d y over denominator d x end fraction minus 2 x y equals 0


    Let the center be at point (0,a).
    So, the equation of the circle with its center on y axis will be 
    (x - 0)2 + (y - a)2 = a2
    As the center is at distance a from origin and the circle is touching the origin, it's radius will be a.
    The equation is
    x2 + (y - a)2 = a2
    We will expand the equation.
    x2 + y2 - 2ay + a2 = a2
    x+ y2 - 2ay = 0          ...(1)
    We will differentiate the above equation.
    2x + 2yfraction numerator d y over denominator d x end fraction - 2afraction numerator d y over denominator d x end fraction = 0.
    2x + 2yfraction numerator d y over denominator d x end fraction = 2afraction numerator d y over denominator d x end fraction.            ...(2)
    We will find the value of 2a from equation (1)
    x squared plus y squared minus 2 a y space equals space 0
2 a space equals space fraction numerator x squared plus y squared over denominator y end fraction
    We will substitute this value in equation (2)
    2 x space plus space 2 y fraction numerator d y over denominator d x end fraction equals 2 a fraction numerator d y over denominator d x end fraction
2 x space plus space 2 y fraction numerator d y over denominator d x end fraction equals fraction numerator x squared plus y squared over denominator y end fraction fraction numerator d y over denominator d x end fraction
2 x space plus space 2 y fraction numerator d y over denominator d x end fraction minus fraction numerator x squared plus y squared over denominator y end fraction fraction numerator d y over denominator d x end fraction equals 0
2 x space plus space open parentheses 2 y space minus space fraction numerator x squared plus y squared over denominator y end fraction close parentheses fraction numerator d y over denominator d x end fraction equals 0
2 x space plus space fraction numerator 2 y squared minus x squared plus y squared over denominator y end fraction fraction numerator d y over denominator d x end fraction equals 0
2 x space plus space fraction numerator y squared minus x squared over denominator y end fraction fraction numerator d y over denominator d x end fraction equals 0
M u l t i p l y i n g space b o t h space t h e space s i d e s space b y space minus y space a n d space r e a r r a n g i n g
left parenthesis x squared minus y squared right parenthesis fraction numerator d y over denominator d x end fraction minus 2 x y space equals space 0
    This is the required differential equation of family of circles with center at y-axis and touching the origin.





     
     

    For such questions, we should know the equation of cricle with its centre at a point other than origin.

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