You can solve this problem by using the Gauss method. The Gauss method is a method where you can add the first and last numbers together, and then multiply by how many times that appears. Here, the sum of the first and last number is -11. In this sequence, it would appear ten times (since there are ten terms and the terms are used twice in every eleven). -11 * 5 is -55, so that means the answer to this question is -55.
21:3
9:3
3:1
there all a 3:1 ratio
<span>-1<0
so use 2x+1
2(-1)+1=-2+1=-1
-
</span><span>F(-1) means "Evaluate F(x) when x=-1"
Since -1 is less than 0, this means that F(x) = 2x+1. This is because F(x) = 2x+1 when x < 0</span>