# Why is it useful to isolate the highest order derivative when solving DEs?

1. Apr 3, 2012

### cocopops12

i'm studying differential equations, the book states the following:

i want to know what are some of the "theoretical and computational purposes" behind it?

i always notice the book likes to make the coefficient of the highest derivative equal to 1
but why is that?!

2. Apr 3, 2012

### Office_Shredder

Staff Emeritus
If you've done Euler's method or separation of variables, you saw in the first order case how you have to solve for the derivative as a function of x and y. This extends to higher orders... suppose we have a second order differential equation

$$\frac{d^2 y}{dx^2} = x*y + \frac{dy}{dx}$$
and y(0)=1, y'(0)=1

I want to do an Euler's method kind of calculation. This requires knowing the derivative at every step. Unfortunately I don't have a formula for the derivative, so I have to use the second derivative to re-calculate the derivative each time

y(.1)=y(0)+.1*y'(0) = 1.1
y'(.1)=y'(0)+.1*y''(0)
I know what y''(0) is from the differential equation: y''(0)=0*1+1=1
y'(.1)=1+.1*1=1.1

Now if I want to update to get y(.2) and y'(.2), I need to know what y''(.1) is. So I use the differential equation
y''(.1) = xy+y' = .1*1.1+1.1 = 1.21

Now I can go to x=.2
y(.2)=y(.1)+.1*y'(.1)=1.21
y'(.2)=y'(.1)+.1*y''(.1)=1.221

Now I need to know what y''(.2) is because I want to be able to calculate y and y' at x=.3, etc. So

y''(.2)=x*y+y' = .2*1.21+1.221 = 1.463

Now I can keep going, estimating y(x) for as large a value of x as I want. The computation only required being able to solve for the second derivative of y at each step, since I already knew all the lower derivatives, so it was useful to have the equation in the form y''=f(x,y,y')

3. Apr 3, 2012

### cocopops12

thank you sir!, haven't studied Euler's Method yet, but i do get the idea.