I thought I was supposed to substitute the A value back into the given equation.
i.e. I thought that since
A=(x2+y2), I should substitute the differential of A,
(x2+y2)dxdyI was under the impression this was what they did in the worked example in my book, when finding a differential equation for
x2+y2=AyFirst they differentiate with respect to x:
2x+2ydxdy=AdxdyThen multiply by y:
2xy+2y2dxdy=AydxdyAnd then using the given equation:
2xy+2y2dxdy=(x2+y2)dxdy