For the equation y″ + 5y′ + 6y = 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″ + 5y′ + 6y = 0
r =
2.Easy
A radioactive tracer has a half-life of 3 days. A sample initially contains 100 mg. The decay follows dm/dt = −k·m. How much m remains after 8 days?
m0 = 100 mgt½ = 3 dt = 8 d
m = mg
3.Medium
Solve y″ + 2y′ + y = 0 with y(0) = 5 and y′(0) = 2. Find y(1.5).
y″ + 2y′ + y = 0y(0) = 5y′(0) = 2
y(1.5) =
4.Medium
A forged steel part at 320 °C is left to cool in a workshop at 21 °C. After 46 min its temperature is 140 °C. It follows Newton's law of cooling, dT/dt = −k·(T − Tₐ). Find the constant k in min⁻¹.
T0 = 320 °CTa = 21 °CT(46) = 140 °C
k = min⁻¹
5.Hard
Solve y″ + 2y′ + 10y = 0 with y(0) = 2 and y′(0) = 0 (released from rest). Find y(1.5); angles in radians.
y″ + 2y′ + 10y = 0y(0) = 2y′(0) = 0
y(1.5) =
6.Hard
Solve the initial value problem y′ + 3y/x = 4x for x > 0, with y(1) = 2. Find y(2).
y′ + 3y/x = 4xy(1) = 2
y(2) =
Answer key
- 1. r = −2
- 2. m = 15.75 mg
- 3. y(1.5) = 3.459
- 4. k = 0.02003 min⁻¹
- 5. y(1.5) = −0.2395
- 6. y(2) = 3.35
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