Separable First Order ODEs

In the form

y=g(x)h(y)

Alternatively,

dydx=f(x,y);f(x,y)=h(x)g(y)

Eg:
y=xy
y=x2
yxy=xy=x(y+1)

Not SDE:
y=x2+y2

  1. Constant solutions: h(y)=0
    • y=F(x,y)
    • F(x,a)=0 for all x
    • y=a is a constant solution
  2. Non-constant solutions:
dydx=g(x)h(y)dyh(y)=g(x)dxdyh(y)=g(x)dx+c

Solution Technique

  1. Rewrite as 1g(y)dy=h(x)dx (provided g(y)0)
  2. Integrate both sides

Implicit check:

  1. Implicitly differentiate both sides
  2. Rearrange for y
  3. Verify that it matches the original ODE
    See also: Implicit Differentiation

Applications of Separable ODEs