## kiara23 3 years ago PLEASE HELP URGENT ! what is the 7th of the geometric sequence where a1 = 3 and a5 = -768

1. amistre64

nothings THAT urgent :/

2. amistre64

we should start by remembering the general formula for a geometric sequence

3. amistre64

an = a1*r^(n-1)

4. amistre64

we know a1, and we know the value when n=5; from this we can grab the r value

5. amistre64

can you fill those in for me?

6. kiara23

kind of

7. amistre64

give it a try, ill know if its right or not :)

8. kiara23

the answer is going to be -3,072

9. amistre64

it might be, but lets see if we can build up to that

10. kiara23

ok

11. amistre64

lets fill in what we are given: a1 = 3 and a5 = -768 when n=5 an = a1 * r^(n-1) a5 = -786 = 3 * r^(5-1) -786/3 = r^(4) -292 = r^(4) , 4rt the whole thing to find r 4rt(-292) = r as is this is not a possible sequence to me. an even root can be negative across the reals

12. amistre64

an even root canNOT .... i passed typing class, honest :)

13. amistre64

check your problem and make sure you havent got an error in the post

14. kiara23

ok

15. amistre64

i got a feeling the a1 = -3 for this to work like you want it to

16. kiara23

yeah

17. kiara23

it is

18. amistre64

4rt(292) = r then this is what we get for r take the general formula again, knowing this r and n=7 a7 = -3 * (4rt(292))^(6) and a calculator should popout your intended results

19. kiara23

can it be 12,288

20. amistre64

no

21. amistre64

...can what be?

22. amistre64

a7 is going to be a negative value

23. kiara23

-12,288

24. amistre64

adjust for a mismath and i get: a7 = -3 * 256^(6/4) = -12288 , so yes

25. kiara23

can you help me with thi one ? What is the sum of the geometric sequence -4, 28, -196 ?

26. amistre64

why did you think that the answer "is going to be -3,072"?

27. kiara23

i hd my ngatives wrng

28. kiara23

signs

29. amistre64

ah, itll prolly be best to post up a new question with that sum in it; i have to go for now