What is the sum of the arithmetic sequence that starts with 3 and 9 and ends with 15, with a total of 26 terms?

The sum of an arithmetic sequence can be found using the formula:

S_n = (n/2) * (a + l)

Where:

• S_n = Sum of the first n terms
• n = Number of terms
• a = First term
• l = Last term

In this case, we have the following:

• a = 3 (the first term)
• l = 15 (the last term)
• n = 26 (the number of terms)

Plugging in these values, we will first find the sum:

S₂₆ = (26/2) * (3 + 15)

Calculating inside the parentheses:

3 + 15 = 18

Now we substitute that back into the equation:

S₂₆ = 13 * 18

Now calculating that:

S₂₆ = 234

Therefore, the sum of the arithmetic sequence is 234.