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# C2 geometric series Watch

1. Hi. Help on this question, can't seem to get it right.

The second term of a geometric series of positive terms is 100 and the fourth term is 64. Find the number of terms that must be added if the total is to exceed four times the first term.

I have worked out a= 125, r=0.8 and the total must be more than 500. Substitute into formula sn = a(1-r^n)/1-r and taking logs but I can't get the right answer.

Could someone please show me how to do this formula as that is the place I think I'm going wrong.

Thanks.
2. (Original post by HumiT)
Hi. Help on this question, can't seem to get it right.

The second term of a geometric series of positive terms is 100 and the fourth term is 64. Find the number of terms that must be added if the total is to exceed four times the first term.

I have worked out a= 125, r=0.8 and the total must be more than 500. Substitute into formula sn = a(1-r^n)/1-r and taking logs but I can't get the right answer.

Could someone please show me how to do this formula as that is the place I think I'm going wrong.

Thanks.
I can't find a mistake in your working unless you post it
3. (Original post by joostan)
I can't find a mistake in your working unless you post it
I just did my working out again...found my mistake. Thanks anyway
4. (Original post by HumiT)
I just did my working out again...found my mistake. Thanks anyway
Good

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