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core 2 help needed please!

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? :smile:
Reply 1
Avatar for nuodai
nuodai
17
Well you know that, by definition of a geometric sequence, the nnth term is given by un=arn−1u_n = ar^{n-1}. You also know that u2=8u_2=8 and u5=−1u_5=-1. What do you think you can do with these pieces of information?
Reply 2
Avatar for bambamty
bambamty
OP
11
Original post by nuodai
Well you know that, by definition of a geometric sequence, the nnth term is given by un=arn−1u_n = ar^{n-1}. You also know that u2=8u_2=8 and u5=−1u_5=-1. 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
Reply 3
Avatar for nuodai
nuodai
17
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?
Reply 4
Avatar for bambamty
bambamty
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11
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?
Reply 5
Avatar for bambamty
bambamty
OP
11
hmm, i believe it would, i shall go back and re-check that question now :P
Reply 6
Avatar for nuodai
nuodai
17
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?
Reply 7
Avatar for Mr XcX
Mr XcX
15
Hi.

I am having difficulty with these questions too.

Who else is finding core 2 dull compared to Core 1???
Reply 8
Avatar for bambamty
bambamty
OP
11
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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