Problem 1
(1−2)/(2t)=−1/(2t), a function of t, so a t-only integrating factor is possible.
Faculty use
Integrating Factors for Nonexact Equations
Reward correct mathematics, explicit units, conditions, and hypotheses, interpretation, and verification. Equivalent valid reasoning should receive credit.
Problem 2
It converts the form into an exact differential, creating a conserved potential Φ=C.
Problem 3
Definitions should identify the equation type, dependent and independent variables, parameters, conditions, units or domain, and the hypotheses behind finding a multiplier depending on one variable when a derivative ratio simplifies. Use the formulas (M_y−N_t)/N=f(t) ⇒ μ(t)=e^{∫fdt}; (N_t−M_y)/M=g(y) ⇒ μ(y)=e^{∫gdy}.
Problem 4
A strong error analysis identifies the first step that violates classification, algebra, theorem hypotheses, units, initial/boundary data, or numerical assumptions; then repairs that step and re-verifies the final result.
Problem 5
Application responses vary. Require defined variables, a compatible differential-equation model connected to circuits, mixing, heat transfer, finance, chemical processes, and geometric envelopes, a complete computation, interpretation, and limitations.
Problem 6
Accept two independent checks appropriate to the task: differentiation/substitution, initial or boundary conditions, dimensional analysis, phase-line or stability evidence, residuals, step refinement, conservation/energy checks, data comparison, or a second method.
Scoring guidance
Six problems × 10 points: 5 mathematical accuracy, 2 structure/justification, 2 verification/interpretation, 1 notation and communication.