Saturday 29 August 2015

SUM OF A GP 1


The sum of the 1st five terms of a geometrical progression is 484. If the common ratio is 3, find the 1st term.

solution

S5 = 484, G1 = ?, r=3, n=5

Sn = G1(rn-1)/r-1
               

S5 = G1(r5-1)/r-1
               

484= G1(35-1)/3-1
               

484= G1(243-1)
                  2

2 x 484= G1(242)
                  
968= 242 G1

4 968   =  G1
  242

G1 = 4

Hence the 1st term is 4.

TRY THIS……………..


The sum of the 1st seven terms of a geometrical progression is 381. If the common ratio is 2, find the 1st term.



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