Read section 3.4, which provides some shortcuts to 3.5.
Retrieval Practice: Chapter 17 Review (pg. 862): 17, 20, 27, 30, 36, 40.
Assigned HW: 6, 7, 9, 14, 20, 25, 29, 32.
Used to find a particular solution to the non-homogenous equation when you have two solutions $y_1(x)$ and $y_2(x)$ to the correponding homogenous equation
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Variation of Parameters - Nonhomogeneous Second Order Differential Equations
Assigned HW: 51, 52, 53, 55, 59, 62.