Find the sum of the first 20 terms of the sequence - 7, - 3, 1, ...

620

Step-by-step explanation:

The formula to find the sum of the first n terms of an arithmetic sequence is n (a1 + aN) / 2, where n is the number of terms, aN is the last term and a1 is the first term. We do not know the last term, though, so we must find it. The equation for this is an=a1 + (n-1) d, where d is the common difference and a1 is the first term, and n is the number of terms. Therefore, the last term, an is, - 7 + (19) 4 = 69.

Finally, we plug it into the first formula, the formula to find the sum, and we get 20 * (-7+69) / 2, which gives us 620 as our final answer.