Section 1.5 Order
In this section, we’ll explore a key concept in differential equations known as order. Think of the order of a differential equation as the number of layers or steps you need to peel away to reveal the original function. The more layers there are, the higher the order, and usually, the more complex the equation.
Example 9.
\(\ \ \)Give the order of the following differential equations:\(\ds \frac{dy}{dx} + y = x\) 
\(\hspace{24em}\) Solution.
Order = 1. This is a firstorder differential equation because the highest derivative is \(\frac{dy}{dx}\text{.}\)

\(\ds x^2 y'' + y''' = \sin(x) + y^8\) 
\(\hspace{24em}\) Solution.
Order = 3. This is a thirdorder differential equation because the highest derivative is \(y'''\text{.}\)

\(\ds \frac{d^2 A}{dt^2} + \frac{dA}{dt} + A = 17\) 
\(\hspace{24em}\) Solution.
Order = 2 This is a secondorder differential equation because the highest derivative is \(\frac{d^2 A}{dt^2}\text{.}\)

Note 10. Caution, Don’t Confuse Exponents with Derivatives.
It’s important to distinguish between exponents and derivatives. For example, in the second example, \(y^8\) indicates that the dependent variable is raised to the 8th power, but this is not related to the order of the differential equation.
Reading Questions Check your Understanding
1. Which of the following equations is a thirdorder differential equation?
 \(\frac{d^3y}{dx^3} + x^2y = 0\)
 Correct! The highest derivative here is the third derivative, making it a thirdorder differential equation.
 \(\frac{d^2y}{dx^2} + y' = \sin x\)
 Incorrect. This is a secondorder differential equation.
 \(y'' + y' + y = 0\)
 Incorrect. This is a secondorder differential equation.
 \(y' + y = x\)
 Incorrect. This is a firstorder differential equation.
Which of the following equations is a thirdorder differential equation?
2. The order of a differential equation is determined by the number of terms it contains.
 True
 Incorrect. The order is based on the highest derivative, regardless of the number of terms.
 False
 Correct! The order is determined by the highest derivative, not the number of terms.
The order of a differential equation is determined by the number of terms it contains
3. The differential equation \(\ds (1  x)y''  x^4y' + y^5 = \cos x \) has order .
4. The differential equation \(\ds y''' + 3y'  4y = 0 \) has order .
5. The differential equation \(\ds t^3 \frac{d^2y}{dt^2} + \frac{d^5y}{dt^5} = \sin(t) \) has order .
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