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February 13, 2016
Posted by **Jm** on Thursday, January 3, 2013 at 6:06pm.

- Can someone help me?! -
**Jm**, Thursday, January 3, 2013 at 6:13pmMy answer is 12276, is this correct?

- Can someone help me?! -
**drwls**, Thursday, January 3, 2013 at 6:32pmThe arithmetic sequence could be, and is,

12,15,18,21,24,27,30,33,36,39,

42,45,48,51,54,57,60,63,66,69,72

The A1, A5 and A13 terms are 12, 24 and 48. Note that they have the common ratio 2, as required.

The geometric sequence is

12, 24, 36, 72, 144, 288, 576, 1152, 2304, 4608.

It looks like your answer is too large.

- Can someone help me?! -
**Jm**, Thursday, January 3, 2013 at 7:16pmthe sum of the first 10 terms of the geometric sequence.

- Can someone help me?! -
**drwls**, Thursday, January 3, 2013 at 7:21pmI just listed all ten for you. Add them up.

- Can someone help me?! -
**Steve**, Thursday, January 3, 2013 at 7:38pmor, analytically,

a+4d = 2a

a+12d = 4a

a = 4d, so, since a+20d=72

24d=72, and d=3

so, a=12

Tn = 12*2^(n-1)

S10 = 12*(2^10-1)/(2-1) = 12*1023 = 12276

Looks like you're right.

drwls, I think the sequence is

12,24,48,96,... :-(

- Can someone help me?! -
**drwls**, Thursday, January 3, 2013 at 7:42pmSteve is right. I messed up doubling one term. Sorry about that

- Can someone help me?! -
**drwls**, Friday, January 4, 2013 at 6:03am12 + 24 + 48 + 96 +192 +384 +768 + 1536 +3072 + 6144 = 12,276

Congratulations!

I used a brute force method. Steve's is better

- Mathematics -
**Clement**, Friday, February 22, 2013 at 8:16am2+5+8+11+14+17+20......=?

- Can someone help me?! -
**Richard**, Thursday, January 23, 2014 at 6:50pmMath