Engineering
Mathematics
Formation of Differential Equation
Question

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' (herey'=dydx,y''=d2ydx2), then which of the following statement(s) is(are) true?

P = y + x

P – Q = x + y – y' – (y')2.

P + Q = 1 – x + y + y' + (y')2

P = y – x

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Solution

(x – a)2 + (y – a)2 = r2

2(x – a) + 2(y – a)y' = 0

1 + (y – a) y'' + (y')2 = 0

 +(yx+yy'1+y')y'' + (y')2 = 0

 + y' + (y(1+y')    (x+yy'))y''  +(y')2(1+y')= 0

 + (y+yy'    xyy')y''  +y'(1+y'(1+y'))= 0

P = y – x,                      Q = 1 + y' + (y')2

P + Q = y – x + 1 + y' + (y')2