# Sum of Sequence of Products of Consecutive Odd and Consecutive Even Numbers

## Theorem

 $\ds \forall n \in \Z_{>0}: \,$ $\ds \sum_{j \mathop = 1}^n j \paren {j + 2}$ $=$ $\ds 1 \times 3 + 2 \times 4 + 3 \times 5 + \dotsb + n \paren {n + 2}$ $\ds$ $=$ $\ds \frac {n \paren {n + 1} \paren {2 n + 7} } 6$

## Proof

The proof proceeds by induction.

For all $n \in \Z_{>0}$, let $\map P n$ be the proposition:

$\ds \sum_{j \mathop = 1}^n j \paren {j + 2} = \frac {n \paren {n + 1} \paren {2 n + 7} } 6$

### Basis for the Induction

$\map P 1$ is the case:

 $\ds \sum_{j \mathop = 1}^1 j \paren {j + 2}$ $=$ $\ds 1 \times 3$ $\ds$ $=$ $\ds 3$ $\ds$ $=$ $\ds \frac {1 \paren {1 + 1} \paren {2 \times 1 + 7} } 6$

Thus $\map P 1$ is seen to hold.

This is the basis for the induction.

### Induction Hypothesis

Now it needs to be shown that, if $\map P k$ is true, where $k \ge 1$, then it logically follows that $\map P {k + 1}$ is true.

So this is the induction hypothesis:

$\ds \sum_{j \mathop = 1}^k j \paren {j + 2} = \frac {k \paren {k + 1} \paren {2 k + 7} } 6$

from which it is to be shown that:

$\ds \sum_{j \mathop = 1}^{k + 1} j \paren {j + 2} = \frac {\paren {k + 1} \paren {k + 2} \paren {2 \paren {k + 1} + 7} } 6$

### Induction Step

This is the induction step:

 $\ds \sum_{j \mathop = 1}^{k + 1} j \paren {j + 2}$ $=$ $\ds \sum_{j \mathop = 1}^k j \paren {j + 2} + \paren {k + 1} \paren {k + 3}$ $\ds$ $=$ $\ds \frac {k \paren {k + 1} \paren {2 k + 7} } 6 + \paren {k + 1} \paren {k + 3}$ Induction Hypothesis $\ds$ $=$ $\ds \paren {k + 1} \paren {\frac {k \paren {2 k + 7} + 6 \paren {k + 3} } 6}$ $\ds$ $=$ $\ds \paren {k + 1} \paren {\frac {2 k^2 + 13 k + 18} 6 }$ $\ds$ $=$ $\ds \frac {\paren {k + 1} \paren {k + 2} \paren {2 k + 9} } 6$ $\ds$ $=$ $\ds \frac {\paren {k + 1} \paren {k + 2} \paren {2 \paren {k + 1} + 7} } 6$

So $\map P k \implies \map P {k + 1}$ and the result follows by the Principle of Mathematical Induction.

Therefore:

$\ds \forall n \in \Z_{>0}: \sum_{j \mathop = 1}^n j \paren {j + 2} = \frac {n \paren {n + 1} \paren {2 n + 7} } 6$

$\blacksquare$