Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2022/ia/paper-2/7c/b/solution

Resolve the forcing in the eigenbasis:
In the zero-eigenvalue direction, a constant forcing produces the particular integral . In the unit-eigenvalue direction, a constant particular integral is . Thus a particular integral depends on time exactly when
Adding the complementary solution from part (a), the general solution is
This is the eigenvector form of the usual variation of parameters calculation.

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