arithmetic/geometric sequences, HELP NEEDED BADLY |
 Sep 18 2005, 01:39 AM
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 in a matter of time  Group: Staff Alumni Posts: 7,151 Joined: Aug 2005 Member No: 191,357  |
Yeah. Both my friend and I are totally stuck on this.
3+7+11+15... How many terms have a sum of less than 500? Formula for general term (where n=# of terms, a=first term, d=common difference): Term of n = a+(n-1)d Okay. So you need to find the general term first, right? Term of n = 3+(n-1)4 Term of n = 4n-1 Formula for sum of arithmetic sequence (where n=# of terms, a=first term, d=common difference): Sum of n = n/2(a+term of n) So that would be... Sum of n = n/2(3+4n-1) Sum of n = n/2(4n+2) Sum of n = 2(n^2)+n To find the terms with sum less than 500, you'd go: 2(n^2)+n < 500 ...right? The thing is. How do you solve that? -------------------------- Second question. How do you solve for "n" in: 3^(n-1)=729? |
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| *mzkandi* |
Oct 6 2005, 01:29 PM
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#2
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This is kind old..Gigi will re-open it if she needs to.
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Posts in this topic
gigiopolis arithmetic/geometric sequences Sep 18 2005, 01:39 AM
bonluvdan for the second question, simply law of indices
3^... Sep 18 2005, 01:56 AM kryogenix 2n^2+n < 500
n(2n+1) < 500 ?
I guess you... Sep 18 2005, 09:14 AM gigiopolis Okay, thanks anyway. My friend and I are still clu... Sep 22 2005, 12:40 AM Olive I tried the first qn but I did it different to you... Sep 25 2005, 07:17 AM FREEcandies She has all the equations (or inequations) right, ... Sep 25 2005, 04:14 PM warriors1035 HOLY SHIT wat grade arre all of u in
im in 7th an... Sep 26 2005, 02:57 PM gigiopolis This is 10 Honours.
I'm actually in 11 Honour... Sep 27 2005, 12:14 AM
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