Question

1.
First term and common difference of an A.P. are 15 and -5 respectively.
Find S13.
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Answer 1

Step-by-step explanation:

t1 = a = 15

common difference = d = -5

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

S13 = 13\2 (2×15+(13-1)(-5)

= 13\2 (30+12×(-5)

= 13\2 (30+(-60))

= 13\2 (30)

= 13\2×30

= 390\2

S13 = 195

hope it will help you