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Question: Answered & Verified by Expert
If l and m are the degree and the order respectively of the differential equation of the family of all circles in the XY plane with radius 5 units, then 2l+3m=
MathematicsDifferential EquationsAP EAMCETAP EAMCET 2018 (25 Apr Shift 1)
Options:
  • A 5
  • B 10
  • C 15
  • D 7
Solution:
2376 Upvotes Verified Answer
The correct answer is: 10

Let the coordinates of the centre of the circle be h,k and radius be r. Then, equation of circle is

x-h2+y-k2=52   ..1

Differentiating both sides w.r.t. x, we get

2x-h+2y-ky'=0

x-h=-y-ky'  ...i

Again differentiating w.r.t. x, we get

1=-y'2+y-ky''

y-k=-1+y'2y''   ...ii

Now, equation i becomes 

x-h=1+y'2y''y'  ...iii

Substituting the values of x-h & y-k from ii & iii in 1, we get

1+y'2y''y'2+-1+y'2y''2=25

1+y'23y''2=25

1+y'23=25y''2

Highest order derivative is of order 2. Therefore, order of differential equation is 2. Hence,

m=2

Highest order derivative is of order 2 whose degree is 2. Therefore, degree of differential equation is 2. Thus,

l=2

And,

2l+3m=10

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