## Val97 2 years ago Please Help! Show me how its done please! Find the sum of the first 8 terms of the sequence. 1, -3, -7, -11, ...

1. amistre64

sooo, you need help adding? or is it that you really need help in determining all the 8 terms?

2. amistre64

since the numbers are getting smaller, or going negative, ... and they seem fairly close together. We can try to see if they are subtracting a like amount each time.

3. Notamathgenius

Fine out what the term is going up by. I see -4

4. Val97

I need help determining all the 8 terms, this is pre cal and it really confuses me. I'm not sure what formula to use. The Arithmetic Sequence formula?

5. amistre64

formulas are only half the battle. seeing a pattern is the key once you see that pattern, the formula is known by default

6. Val97

So that is it? Its isn't any of the formulas they gave me?

7. amistre64

if you subtract 4 from the last term given, what do you get?

8. Val97

-15, -19, -23

9. amistre64

.... or if you want to create an expression to use in summation thats fine too $\sum_{n=a}^b~a_1+d(n-1)$ $\sum_{n=a}^b~a_1+\sum_{n=a}^bd(n-1)$ $a_1(b-a)+d\sum_{n=a}^bn-1$ $a_1(b-a)+d\sum_{n=a}^bn-d\sum_{n=a}^b1$ $a_1(b-a)+d\sum_{n=a}^bn-d(b-a)$ stuff like that

10. amistre64

but yes ..., -15, -19, -23, -27 fill in all 8 terms

11. amistre64

$\frac{n}{2}(a_1+a_8)=sum$

12. Val97

ok got it! thanks!

13. amistre64

youre welcome, good luck :)