For the equation y″ + 6y′ + 8y = 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
1.Easy
y″ + 6y′ + 8y = 0
r =
2.Easy
A radioactive tracer has a half-life of 17 days. A sample initially contains 200 mg. The decay follows dm/dt = −k·m. How much m remains after 6 days?
m0 = 200 mgt½ = 17 dt = 6 d
m = mg
3.Medium
A forged steel part at 300 °C is left to cool in a workshop at 25 °C. After 48 min its temperature is 50 °C. It follows Newton's law of cooling, dT/dt = −k·(T − Tₐ). Find the constant k in min⁻¹.
T0 = 300 °CTa = 25 °CT(48) = 50 °C
k = min⁻¹
4.Medium
Solve y″ + 7y′ + 12y = 0 with y(0) = 2 and y′(0) = 1. Find y(1.5).
y″ + 7y′ + 12y = 0y(0) = 2y′(0) = 1
y(1.5) =
5.Hard
A system obeys y″ + 3y′ + 11.25y = 22.5 for t ≥ 0, starting from rest: y(0) = 0, y′(0) = 0. Find the peak value ymax of its step response.
y″ + 3y′ + 11.25y = 22.5y(0) = 0y′(0) = 0
ymax =
6.Hard
Solve the initial value problem y′ + y/x = 2x for x > 0, with y(1) = 1. Find y(2).
y′ + y/x = 2xy(1) = 1
y(2) =
Answer key
- 1. r = −2
- 2. m = 156.6 mg
- 3. k = 0.04996 min⁻¹
- 4. y(1.5) = 0.08263
- 5. ymax = 2.416
- 6. y(2) = 2.833
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