Prove 4 + 8 + ... + 4n = 2n(n + 1) by induction Step by Step

Step by step

The question

Question: prove by induction that 4 + 8 + … + 4n = 2n(n + 1).

4 + 8 + … + 4n = 2n(n + 1)

Induction works like dominoes.

Push the first one: it is true for n = 1.

Then show each one knocks over the next: if it is true for k, it is true for k + 1.

Then they all fall.

1 Check n = 1

For n = 1 the left side is just the first number, 4.

The right side with n = 1 is also 4.

The first domino falls.

4 = 2 × 1(1 + 1) = 4

✓ n = 1 works

2 Assume it works for k

Now suppose it works for some whole number k: the sum up to k equals the formula with k.

4 + 8 + … + 4k = 2k(k + 1)

we assume this

3 What we must show

We must show it for k + 1: one more term, 4(k + 1), and every n on the right becomes k + 1.

4 + 8 + … + 4k + 4(k + 1) = 2(k + 1)((k + 1) + 1)

Tidy the brackets.

= 2(k + 1)(k + 2)

we must get this

Careful!

k + 1 goes into every n of the formula.

Adding 1 at the end is not the same.

Careful: not 2k(k + 1) + 1 ✗

4 Use the assumption

The left side starts with the sum up to k.

By our assumption, that is the formula with k.

We swap it in.

4 + 8 + … + 4k + 4(k + 1) = 2k(k + 1) + 4(k + 1)

5 Make both sides the same

Open the brackets.

= 2k² + 2k + 4k + 4

Like terms together.

= 2k² + 6k + 4

Now the right side, the one we must get.

Open the brackets.

2(k + 1)(k + 2) = 2k² + 4k + 2k + 4

Like terms together.

= 2k² + 6k + 4

The same as the left side!

So if it works for k, it works for k + 1.

= the left side ✓

It is true for n = 1, and each n passes it on to the next.

So it is true for every whole number n.

The proof is done.

true for n = 1, and k → k + 1: true for every n ≥ 1. ∎

6 Check

Let's check n = 4 by adding: 4 + 8 + 12 + 16 is 40.

n = 4: 4 + 8 + 12 + 16 = 40

And the formula with 4: also 40.

Right!

2 × 4(4 + 1) = 40

Let's look back.

First: check n = 1.

Then: assume it works for k.

Then: what we must show.

Then: use the assumption.

Then: make both sides the same.

Last: check.

Answer