Posted by Jm on Thursday, January 3, 2013 at 6:06pm.
My answer is 12276, is this correct?
The 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.
the sum of the first 10 terms of the geometric sequence.
I just listed all ten for you. Add them up.
or, 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,... :-(
Steve is right. I messed up doubling one term. Sorry about that
12 + 24 + 48 + 96 +192 +384 +768 + 1536 +3072 + 6144 = 12,276
Congratulations!
I used a brute force method. Steve's is better
2+5+8+11+14+17+20......=?
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