Solve y' + 3y = 6 Step by Step | The integrating factor

Step by step

The question

Question: solve y′ + 3y = 6.

y′ + 3y = 6

1 Standard form

Linear means this form: y′ + P y = Q.

P and Q have only x.

linear: y′ + P(x) y = Q(x)

Here P = 3 and Q = 6.

P = 3, Q = 6

2 The integrating factor

The integrating factor is e to the integral of P.

Multiplying by it makes the left side one derivative.

integrating factor: μ = e^(∫P dx)

First the integral of P: the integral of 3 is 3x.

∫ 3 dx = 3x

So the integrating factor is e^(3x).

No + C needed here: any one factor works.

μ = e^(3x)

3 Multiply both sides

Multiply both sides by e^(3x).

Every term, on both sides.

e^(3x)y′ + 3e^(3x)y = 6e^(3x)

Careful!

The right side is multiplied by e^(3x) too.

Both sides, always.

Careful: not (e^(3x)y)′ = 6 ✗

4 One derivative on the left

By the product rule, the derivative of e^(3x) × y is exactly our left side.

(μy)′ = μy′ + μ′y

μ′ = 3e^(3x)

So the left side is just (e^(3x)y)′: one derivative.

(e^(3x)y)′ = 6e^(3x)

5 Integrate both sides

Integrate both sides.

The integral of a derivative is the function itself.

e^(3x)y = ∫ (6e^(3x)) dx

On the right: 2e^(3x), and + C.

e^(3x)y = 2e^(3x) + C

6 Solve for y

Divide both sides by e^(3x).

y = 2e^(3x) + Ce^(3x)

Divide each term.

Dividing by e to a power is the same as e to minus that power.

y = 2 + Ce^(−3x)

C over e^(3x) = Ce^(−3x)

7 Check

Let's check: put y back in.

First y′ is −3Ce^(−3x).

y′ = −3Ce^(−3x)

Then y′ + 3y.

y′ + 3y = −3Ce^(−3x) + 3(2 + Ce^(−3x))

The C parts cancel, and we get 6: Q again.

Right!

= 6

Let's look back.

First: standard form.

Then: the integrating factor.

Then: multiply both sides.

Then: one derivative on the left.

Then: integrate both sides.

Then: solve for y.

Last: check.

Answer