SOLUTION: The first two terms of a geometric sequence and an arithmetic sequence are the same. The first term is 12. The sum of the first three terms of the geometric sequence is 3 more than

Question 840912: The first two terms of a geometric sequence and an arithmetic sequence are the same. The first term is 12. The sum of the first three terms of the geometric sequence is 3 more than the sum of the first three terms of the arithmetic sequence. Determine TWO possible values for the common ratio, r, of the geometric sequence
Answer by Edwin McCravy(20056) (Show Source): You can put this solution on YOUR website!
The first two terms of a geometric sequence and an arithmetic sequence are the same. The first term is 12. The sum of the first three terms of the geometric sequence is 3 more than the sum of the first three terms of the arithmetic sequence. Determine TWO possible values for the common ratio, r, of the geometric sequence

Geometric sequence = a,ar,ar�,__,__,...

Arithmetic sequence= a,a+d,a+2d,__,__,...

The first two terms of a geometric sequence and an arithmetic sequence are the same.

a = a, ar = a+d

The first term is 12. 

a = 12,  12r = 12+d

The sum of the first three terms of the geometric sequence is 3 more than the sum of the first three terms of the arithmetic sequence.

   (a)+(ar)+(ar�) = (a)+(a+d)+(a+2d)+3
(12)+(12r)+(12r�) = (12)+(12+d)+(12+2d)+3
      12+12r+12r� = 12+12+d+12+2d+3
      12+12r+12r� = 39+3d
         12r+12r� = 27+3d

Every term can be divided by 3

           4r+4r� = 9+d

So we have this system of two equations and 2 unknowns:

     12r = 12+d, 4r+4r� = 9+d

Solve the first for d:

         12r = 12+d     
      12r-12 = d

Substitute in

      4r+4r� = 9+d
      4r+4r� = 9+(12r-12)
      4r+4r� = 9+12r-12
      4r+4r� = 12r-3
    4r�-8r+3 = 0
(2r-1)(2r-3) = 0
   2r-1=0;  2r-3=0
     2r=1;    2r=3
      r=;    r=

Those are the two values.

---------------------------------------

To check we must find d

Using r = 

 12r-12 = d
12-12 = d
   6-12 = d
     -6 = d

Arithmetic sequence:  12, 6, 0, ...
Geometric sequence:   12, 6, 3, ...

Sum of 1st three terms of arithmetic sequence = 12+6+0 = 18
Sum of 1st three terms of geometric sequence  = 12+6+3 = 21
21 is 3 more than 18
That checks.

Using r = 

 12r-12 = d
12-12 = d
  18-12 = d
      6 = d

Arithmetic sequence:  12, 18, 24, ...
Geometric sequence:   12, 18, 27, ...

Sum of 1st three terms of arithmetic sequence = 12+18+24 = 54
Sum of 1st three terms of geometric sequence  = 12+18+27 = 57
57 is 3 more than 54
That checks, too.  So it's correct

Edwin

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