Thursday 19 December 2013

SEQUENCE AND SERIES A5

The sum of the 1st six terms of a geometrical progression is 728. If the common ratio is 3 find the 1st term.

solution

S6 = 484 G1 = ?, r=3, n=5

Sn = G1(rn-1)/r-1
               

S6 = G1(r6-1)/r-1
               

728= G1(36-1)/3-1
               

728= G1(729-1)
                  2

2 x 728= G1(728)
                  
1456= 728G1

2 1456   =  G1
  728

G1 = 2


Hence the 1st term is 2.

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