a new sequence is formed when adding corresponding terms of a geometric sequence and an arithmetic sequence. The G.S has a common ratio of 3 and the A.S a common difference of -2. first two terms of the new sequence are 4 and 20. calculate term 3 of the new sequence...please help
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2 Answers
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If I well understand your question the geometric progression is: $$ a\;,\;3a\;,\;9a\;,\;27 a\;,\;\cdots $$ and the arithmetic progression is $$ b\;,\;b-2\;,\;b-4\;,\;b-6\;,\;\cdots $$
so, adding the corresponding terms $ a_1+b_1$ and $a_2+b_2$ we have
$$ a+b=4 \qquad 3a+b-2=20 $$
solving the system of the two equation you can find $a,b$ and the third term.
answered Mar 14, 2016 at 21:19
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If $a_1, a_2$ are the first two numbers in the arithmetic sequence and $g_1, g_2$ are the first two numbers in the geometric sequence. We have 4 unknowns. Can we create 4 equations based on the given information, and use that to solve for the 4 unknowns?
$a_1+g_1 = 4; a_2+g_2 = 20; a_2 = a_1 - 2; g_2 = 3 g_1$
That is enough information to solve for $a_1,g_1$ and you have the rules to find the next terms in the each sequence.
