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## Main Question or Discussion Point

There is a specific step when solving first-order equations introduced in my textbook when describing the process for solving with an integrating factor, and it has been popping up everywhere - I do not understand how it works and the book doesn't explain it well.

Example 1:

x(dy/dx) + y = sin(x) / x

becomes

d/dx(xy) = sin(x) / x

Example 2:

e^-x (dy/dx) - y(e^-x) = e^(-4x/3)

becomes

d/dx(y(e^-x)) = e^(-4x/3)

I'm having trouble rationalizing how this step is performed. Thanks for any help!

Example 1:

x(dy/dx) + y = sin(x) / x

becomes

d/dx(xy) = sin(x) / x

Example 2:

e^-x (dy/dx) - y(e^-x) = e^(-4x/3)

becomes

d/dx(y(e^-x)) = e^(-4x/3)

I'm having trouble rationalizing how this step is performed. Thanks for any help!