Edexcel A Level Further Maths: Core Pure:复习笔记8.1.2 Solving First Order Differential Equations

What is a differential equation?

• A differential equation is simply an equation that contains derivatives
  • For example  is a differential equation
  • And so is

What is a first order differential equation?

• A first order differential equation is a differential equation that contains first derivatives but no second (or higher) derivatives
  • For example  is a first order differential equation
  • But  is not a first order differential equation, because it contains the second derivative
• The general solution to a first order differential equation will have one unknown constant
• To find the particular solution you will need to know an initial condition or a boundary condition

Wait – haven’t I seen first order differential equations before?

• Yes you have!
  • For example  is also a first order differential equation, because it contains a first derivative and no second (or higher) derivatives
  • But for that equation you can just integrate to find the solution y = x3 + c (where c is a constant of integration)
• In A Level Maths you will have solved some first order differential equations using the method of separation of variables

What is an integrating factor?

• An integrating factor can be used to solve a differential equation that can be written in the standard form
  • Be careful – the ‘functions of x’ p(x) and q(x) may just be constants!
    • For example in , p(x) = 6 and q(x) = e-2x
    • While in   and q(x) = 12
• For an equation in standard form, the integrating factor is

How do I use an integrating factor to solve a differential equation?

• STEP 1: If necessary, rearrange the differential equation into standard form
• STEP 2: Find the integrating factor
  • Note that you don’t need to include a constant of integration here when you integrate  ∫p(x) dx
• STEP 3: Multiply both sides of the differential equation by the integrating factor

• This will turn the equation into an exact differential equation of the form
• STEP 4: Integrate both sides of the equation with respect to x
  • The left side will automatically integrate to
  • For the right side, integrate  using your usual techniques for integration
  • Don’t forget to include a constant of integration
    • Although there are two integrals, you only need to include one constant of integration
• STEP 5: Rearrange your solution to get it in the form y = f(x)

What else should I know about using an integrating factor to solve differential equations?

• After finding the general solution using the steps above you may be asked to do other things with the solution
  • For example you may be asked to find the solution corresponding to certain initial or boundary conditions