Question

find the sum of 20 terms of the series 2 - 4 + 6 - 8........

Answer 1

(2-4)+(6-8).....is the correct series
(-2)+(-2).....
a1 = first term i.e.-2
common difference is a2-a1 =(-2)-(-2)
=0
n = 20

we know that

Sum=
$\frac{n}{2} (2a + (n - 1)d)$

=10(-4+20×0)
=10×-4
=-40

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