Prove: The square of a number that is one more than a multiple of 3 is also one more than a multiple of 3.

[tex]\[
\begin{aligned}
(3n+1)^2 &= 9n^2 + 6n + 1 \\
&= 3(3n^2 + 2n) + 1 \\
&= 1 \text{ more than a multiple of } 3
\end{aligned}
\][/tex]



Answer :

Certainly! Let's work through the proof step-by-step.

Given expression: [tex]\((3n + 1)^2\)[/tex]

We need to expand and simplify this expression to prove the statement.

### Step 1: Expand the expression
Let's expand the square:

[tex]\[ (3n + 1)^2 = (3n + 1)(3n + 1) \][/tex]

Applying the distributive property (FOIL method):

[tex]\[ (3n + 1)(3n + 1) = 3n \cdot 3n + 3n \cdot 1 + 1 \cdot 3n + 1 \cdot 1 \][/tex]

[tex]\[ = 9n^2 + 3n + 3n + 1 \][/tex]

[tex]\[ = 9n^2 + 6n + 1 \][/tex]

### Step 2: Identify the coefficients
From the expanded expression [tex]\(9n^2 + 6n + 1\)[/tex]:

[tex]\[ \begin{aligned} n^2 \text{ term: } & 9n^2, \quad \text{(coefficient is 9)} \\ n \text{ term: } & 6n, \quad \text{(coefficient is 6)} \\ \text{constant term: } & 1 \end{aligned} \][/tex]

Thus, we can write:

[tex]\[ (3n + 1)^2 = 9n^2 + 6n + 1 \][/tex]

### Step 3: Factor out a multiple of 3 and simplify
We notice that [tex]\(9n^2 + 6n\)[/tex] is a multiple of 3:

[tex]\[ 9n^2 + 6n = 3(3n^2 + 2n) \][/tex]

So, we can rewrite the expression as:

[tex]\[ 9n^2 + 6n + 1 = 3(3n^2 + 2n) + 1 \][/tex]

This shows that [tex]\(9n^2 + 6n\)[/tex] is a multiple of 3, and when we add 1, we get a number that is one more than a multiple of 3.

### Conclusion
[tex]\[ \begin{aligned} (3n + 1)^2 &= 9n^2 + 6n + 1 \\ &= 3(3n^2 + 2n) + 1 \end{aligned} \][/tex]

This confirms that [tex]\((3n + 1)^2\)[/tex] is indeed one more than a multiple of 3.

Therefore, we have proven that the square of a number that is one more than a multiple of 3 is also one more than a multiple of 3.