Step by step
The question
Question: solve y″ + 25y = 0.
1 Try y = e to the rx
The derivatives of e to the rx are e to the rx again, times r.
So we try y = e^(rx), and look for the right r.
Its derivative: r e^(rx).
Once more: r² e^(rx).
2 The characteristic equation
Put them into the equation.
Every term has e^(rx): take it out.
e to a power is never 0.
So the bracket must be 0.
r² + 25 = 0: the characteristic equation.
y″ became r², y′ became r, y became 1.
3 Solve for r
It does not factor nicely, so we use the formula, with a = 1, b = 0, c = 25.
Put the numbers in.
Under the root: −100.
A minus under the root: √(−100) = 10i, because i = √(−1).
Divide both parts by 2: r = ±5i.
Two complex roots.
Careful!
Use the root, 5, not the number under it.
4 Write the solution
Complex roots give cosines and sines.
The real part 0 goes in the e, the 5 goes inside cos and sin.
So y = C₁cos(5x) + C₂sin(5x).
5 Check
Let's check one root, 5i.
Square it, with i² = −1: −25.
Into r² + 25: the numbers cancel and the i parts cancel.
0.
Right!
Let's look back.
First: try y = e to the rx.
Then: the characteristic equation.
Then: solve for r.
Then: write the solution.
Last: check.