Let y=y(x) be the solution of the differential equation x ( yx )dy= (y ( yx )-x )dx, y(1)= 2 and let = ( y (e12 )e12 ). Then the number of integral values of p, for which the equation x2+y2-2px+2py+ +2=0 represents a circle of radius r 6, is .
Answer
6
Solution
The equation is homogeneous, so put y = vx, dy = v dx + x dv:
x v (v dx + x dv) = (vx v - x) dx
x2 v dv = -x dx v dv = - dxx
- v = - e x + C
y(1) = 2 means v = 2 at x = 1, so C = 0 and ( yx ) = e x. Hence
= ( y (e12 )e12 ) = e e12 = 12
The circle is then x2 + y2 - 2px + 2py + 14 = 0, with centre (p, -p) and
r2 = p2 + p2 - 14 = 2p2 - 14
For a real circle r2 > 0, so 2p2 - 14 > 0 p2 > 7; and r 6 gives 2p2 - 14 36 p2 25.
Hence 7 < p2 25, i.e. p = 3, 4, 5.
∴ Number of integral values of p = 6
Answer: 6