1.

Let F be the family of ellipse whose centre is the origin and major axis is the y-axis. Then the differential equation of family F isA. `(d^(2)y)/(dx^(2))+(dy)/(dx)(x(dy)/(dx)-y)=0`B. `xy(d^(2)y)/(dx^(2))-(dy)/(dx)(x(dy)/(dx)-y)=0`C. `xy(d^(2)y)/(dx^(2))+(dy)/(dx)(x(dy)/(dx)-y)=0`D. `(d^(2)y)/(dx^(2))-(dy)/(dx)(x(dy)/(dx)-y)=0`

Answer» Correct Answer - A
Let the equation of the family F of required ellipses be
`Ax^(2)+By^(2)=1" …(i)"`
It is a two parameter family of curves.
Differentiating (i) with respect to x, we get
`ax+Byy_(1)=0" …(ii)"`
Differentiating (ii) with respect to x, we get
`A+By_(1)^(2)+Byy_(2)=0" ...(iii)"`
Multiplying (iii) by x and subtracting from (ii), we get
`B[yy_(1)-xy_(1)^(2)-xyy_(2)]=0`
`rArr" "xyy_(2)+y_(1)(xy_(1)-y)=0rArrxy(d^(2)y)/(dx^(2))+(dy)/(dx)(x(dy)/(dx)-y)=0`
This is the required differential equation.


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