Concept: Convert the given slope into a linear differential equation in
x and solve using an integrating factor.
Explanation:The slope of the tangent at
P(x,y) is
dxdy=x+yy.
Since the denominator contains
x+y, write the equation in terms of
dydx.
Taking reciprocal gives
dydx=yx+y=yx+1.
Rearrange to get
dydx−yx=1.
This is a linear differential equation in
x.
Its integrating factor is
e∫−y1dy=e−lny=y1.
Multiplying throughout by
y1, we obtain
y1dydx−y2x=y1.
The left side is
dyd(yx), so
dyd(yx)=y1.
Integrating both sides:
yx=lny+C.
The curve passes through
(0,1), so put
x=0,
y=1:
0=ln1+C gives
C=0.
Thus
yx=lny, which in exponential form is
y=ex/y.
Answer: Option D:
y=eyx