Saturday, 21 December 2013

SEQUENCE AND SERIES A11

The sum of the 1st six terms of a geometrical progression is 189. If the common ratio is 2 find the 1st term.

solution

S6 = 189 G1 = ?, r=2, n=6

Sn = G1(rn-1)/r-1
               

S6 = G1(r6-1)/ r-1
              

189= G1(26-1)/2-1
               

189= G1(64-1)
                  1

189= G1(63)

                 
189 = 63 G1

3 189   =  G1
   63

G1 = 3


Hence the 1st term is 3.
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