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
1.Easy
y″ + 4y′ + 3y = 0
r =
2.Easy
A radioactive tracer has a half-life of 8 days. A sample initially contains 120 mg. The decay follows dm/dt = −k·m. How much m remains after 30 days?
m0 = 120 mgt½ = 8 dt = 30 d
m = mg
3.Medium
Solve y″ + 8y′ + 15y = 0 with y(0) = 1 and y′(0) = 6. Find y(0.5).
y″ + 8y′ + 15y = 0y(0) = 1y′(0) = 6
y(0.5) =
4.Medium
A cup of coffee at 95 °C is left in a room at 19 °C. After 16 min its temperature is 37 °C. It follows Newton's law of cooling, dT/dt = −k·(T − Tₐ). How long after the start, t, does it take to reach 27 °C?
T0 = 95 °CTa = 19 °CT(16) = 37 °CT = 27 °C
t = min
5.Hard
Solve y′ + 0.5y = 8e−3t with y(0) = 1. Find y(1).
y′ + 0.5y = 8e−3ty(0) = 1
y(1) =
6.Hard
Solve the initial value problem y′ + y/x = 2x2 for x > 0, with y(1) = −3. Find y(3).
y′ + y/x = 2x2y(1) = −3
y(3) =
Answer key
- 1. r = −1
- 2. m = 8.919 mg
- 3. y(0.5) = 0.8578
- 4. t = 25.01 min
- 5. y(1) = 2.388
- 6. y(3) = 12.33
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