Question

Find the sum of the first 18 terms of an ap 12b,8b ,4b....

Answer 1

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Question : Find the sum of the first 18 terms of an ap 12b,8b ,4b....

Answer :

$s_{18}=-396b$

Step-by-step explanation :

Series is 12b , 8b , 4b , ....

Here , First term , a = 12b

Common difference , d = 8b - 12b = -4b

$So,s_n=\frac{n}{2}[2a+(n-1)d] \\\\=>s_{18}=\frac{18}{2}[2(12b)+(18-1)(-4b)]\\\\=>s_{18}=9[24b-68b]\\\\=>s_{18}=-396b$

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