$\"\"$

$\"\"$ and $\"\"$ .

Find $\"\"$

The general form of partial sum of $\"\"$ -terms $\"\"$ .

Substitute $\"\"$ and $\"\"$ in the general form.

$\"\"$

The general form of $\"\"$ -terms of arthmetic sequence $\"\"$

Substitute $\"\"$ and $\"\"$ in the general form.

$\"\"$

The value of $\"\"$ is $\"\"$ .

$\"\"$

The value of $\"\"$ is $\"\"$ .