Class 11 Maths Sequences Series Sum of terms of a GP

Sum of terms of a GP:

Sn = a (1-rn) /(1-r)                          or                   Sn = a (rn -1)  /(r-1)

Refer ExamFear video lessons for Proof.

Numerical: Find sum of first 5 terms of geometric series 1,  2/3,  4/9 ….

Solution: in this case a=1, r = 2/3

Sn = a (1-rn) /(1-r)                         

S5 = a (1-r5) /(1-r)      

Or   S5 = 1 ( 1 – (2/3)5 )/(1-2/3)            =  211/81

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