$\"\"$

The values are $\"\"$ .

Apply sum of an arithmetic series formula: $\"\"$ .

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

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

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

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

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

$\"\"$ (Divide: $\"\"$ ) $\"\"$

The value of $\"\"$ .