For the equation y″ + 4y′ + 3y = 0, find the root r of the characteristic equation that is closest to zero. It sets how slowly the solution dies out.
Differential equations worksheet with answers
A radioactive tracer has a half-life of 5 days. A sample initially contains 20 mg. The decay follows dm/dt = −k·m. How much m remains after 8 days?
A can of soda at 4 °C is taken out of the fridge into a room at 28 °C. After 27 min its temperature is 23 °C. It follows Newton's law of cooling, dT/dt = −k·(T − Tₐ). Find the constant k in min⁻¹.
A can of soda at 6 °C is taken out of the fridge into a room at 29 °C. It follows Newton's law of cooling, dT/dt = −k·(T − Tₐ). With k = 0.07 min⁻¹, find its temperature T after 23 min.
A system obeys y″ + y′ + 16.25y = 16.25 for t ≥ 0, starting from rest: y(0) = 0, y′(0) = 0. Find the peak value ymax of its step response.
Solve the initial value problem y′ + 3y/x = 4 for x > 0, with y(1) = 2. Find y(3).
Answer key
- 1. r = −1
- 2. m = 6.598 mg
- 3. k = 0.0581 min⁻¹
- 4. T = 24.4 °C
- 5. ymax = 1.675
- 6. y(3) = 3.037
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