$\"\"$

The values are $\"\"$ .

Where $\"\"$ is the first term and $\"\"$ is the nth term.

The sum of an arithmetic series $\"\"$ .

$\"\"$ (Substitute $\"\"$ )

$\"\"$ (Subtract: $\"\"$ )

$\"\"$ (Multiply: $\"\"$ )

$\"\"$ (Subtract: $\"\"$ )

$\"\"$ (Cancel common terms) $\"\"$

The value of $\"\"$ .