Find the sum of the first 20 terms of the arithmetic sequence with first term 8 and common difference -1 .
$s_{20}=\square$ (Type an integer or a decimal.)

Step 1 ：We are given an arithmetic sequence with the first term \(a = 8\) and the common difference \(d = -1\). We are asked to find the sum of the first 20 terms of this sequence.

Step 2 ：The sum of the first \(n\) terms of an arithmetic sequence can be found using the formula: \(s_n = \frac{n}{2} \cdot (a + l)\), where \(s_n\) is the sum of the first \(n\) terms, \(n\) is the number of terms, \(a\) is the first term, and \(l\) is the last term.

Step 3 ：In this case, we know that \(n = 20\), \(a = 8\), and \(d = -1\). We can find the last term using the formula: \(l = a + (n - 1) \cdot d\).

Step 4 ：Substituting the given values into the formula, we get \(l = 8 + (20 - 1) \cdot -1 = -11\).

Step 5 ：Now that we have the last term, we can substitute the values into the sum formula to find the answer: \(s_{20} = \frac{20}{2} \cdot (8 - 11) = -30\).

Step 6 ：So, the sum of the first 20 terms of the arithmetic sequence is \(\boxed{-30}\).