MAT244--2020F > Chapter 2

Lec 0101 - 9/15 Question

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**Brandon Adiwinata**:

I had a question from the lecture about the Methods for solving a differential equation. As indicated in the photo, how do we get u'(sqrt(t^2+1)) = t after plugging in y = u(sqrt(t^2+1)) into the original inhomogeneous equation?

**RunboZhang**:

Hi, I have plugged it in y and calculated the LHS. Computation is attached below.

**Victor Ivrii**:

One should remember that plugging $y=uy_1$ into inhimogeneous equation leaves $u'y_1$ in the left-hand expression. If you do not remember this, therefore you just do not understand the method of variations and you should reread previous slides

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