1, – a, a², – a³ , ... n terms (if a ≠

NCERT Class XI Mathematics - Sequences and Series - Solutions

Question : 43 of 106

Marks: +1, -0

1, – a,

a^2

, –

a^3

, ... n terms (if a ≠ – 1).

Solution:

In the given G.P., a = 1, r = – a and

S_n

\frac{a[1-r^n]}{1-r}

, if |r| < 1

∴

S_n

\frac{[1-(-a)^n]}{1+a}

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