Suppose the solution of the differential equation dydx = (2 + )x - y + 2 x - 2 y - ( - 4 ) represents a circle passing through origin. Then the radius of this circle is :
- 17
- 2
- 172
- 12
Answer
(3) 172
Solution
Cross-multiplying and grouping:
(x dy + y dx) = (2 + )x dx + 2 dx + 2 y dy + ( - 4 )dy
∵ x dy + y dx = d(xy), integrating:
xy = 2 + 2x2 + 2x + y2 + ( - 4 )y + C
A circle has no xy term: = 0
Equal coefficients of x2 and y2: 2 + 2 = = 2
Through origin: C = 0
2x2 + 2y2 + 2x - 8y = 0, i.e. x2 + y2 + x - 4y = 0
∴ Radius = 14 + 4 = 172
Answer: (3)