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JEE Advanced · Mathematics · 29. Differential Eqns

Consider the family of all circles whose centers lie on the straight line y=x. If this family of circles is represented by the differential equation Py" + Qy + 1 = 0,  where P, Q are functions of x, y and y here y= dydx, y" = d2ydx2 , then which of the following statements is (are) true ?

  1. A P=y+x
  2. B P=y-x
  3. C P+Q=1-x+y+y+y2
  4. D P-Q=x+y-y-y2
Verified Solution

Answer & Solution

Correct Answer

(C) P+Q=1-x+y+y+y2

Step-by-step Solution

Detailed explanation

Let required circle be
x2+y2+2gx+y+c=0
Differentiation
2x+2yy+2g1+y=0
x+yy+g1+y=0 ...(i)
Again differentiation
1+yy+y2+gy=0 ...(ii)
Putting value of g in equation (ii),
1+yy+y2-x+yy1+yy=0
So, P=y-x, Q=1+y+y2
i.e., P+Q=1-x+y+yy2
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