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# core 2 help needed please! watch

1. right guys i have the question- the second term of a geometric sequence is 8 and the fifth is -1, could anyone give me an equation i could use to find r and a please?
2. Well you know that, by definition of a geometric sequence, the th term is given by . You also know that and . What do you think you can do with these pieces of information?
3. (Original post by nuodai)
Well you know that, by definition of a geometric sequence, the th term is given by . You also know that and . What do you think you can do with these pieces of information?
i've treated 8 as term one for now, and -1 as term 4, and got the equation -1=8*r^3

so therefore r=-0.5 and a=16, but when i try to prove it, i get the wrong answer, argh
4. (Original post by bambamty)
i've treated 8 as term one for now, and -1 as term 4, and got the equation -1=8*r^3

so therefore r=-0.5 and a=16, but when i try to prove it, i get the wrong answer, argh
Your value for r is correct, but not a. How did you get your value for a?
5. (Original post by nuodai)
Your value for r is correct, but not a. How did you get your value for a?
i put it into the general equation so 8=a*0.5^1
so 8/0.5 = a?
6. hmm, i believe it would, i shall go back and re-check that question now :P
7. (Original post by bambamty)
i put it into the general equation so 8=a*0.5^1
so 8/0.5 = a?
But r isn't 0.5 is it?
8. Hi.

I am having difficulty with these questions too.

Who else is finding core 2 dull compared to Core 1???
9. (Original post by Mewzle)
ar = 8
ar4 = -1
By dividing, r3 = -0.125, and so r = -0.5?
i've got that now! wooo! i just mistyped it on here i think!

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