### Author Topic: 4.2  (Read 1099 times)

#### jeyara85

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• Karma: 0 ##### 4.2
« on: November 13, 2019, 10:42:22 PM »
I was wondering if we will be expected to find the characteristic equations of higher-order equations (3rd,4th, ..., nth), or will we be provided the characteristic equation in the exam.

#### david

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« Reply #1 on: November 14, 2019, 04:09:16 PM »
Finding the characteristic equation for higher order equations is very similar to the second order case.

If we have the differential equation $a_ny^{(n)} + a_{n-1}y^{(n-1)} .... + a_1y' + a_0y = 0$ then the characteristic equation is $a_nr^n + a_{n-1}r^{n-1} .... + a_1r + a_0 = 0$.

To find the roots, we can use the fact that the product of the roots must be  $a_0$ to help guess the roots.
« Last Edit: November 14, 2019, 04:13:43 PM by david »

#### Victor Ivrii ##### Re: 4.2
« Reply #2 on: November 15, 2019, 02:42:45 PM »
Yes, because for equations given they could be found easily

#### ZeCheng Wu

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« Reply #3 on: November 17, 2019, 10:25:51 PM »
what if a(n) is not a constant, like x^2 for example

#### david

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« Reply #4 on: November 18, 2019, 09:36:48 AM »
If any of the $a_i$'s are not constant, then we cannot use the method above. Non-constant coefficient differential equations are generally harder to solve. We discussed a few methods in class such as reduction of order or using the Wronskian, but both methods require already knowing one solution.