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“If S = -- [2a + (n − 1)d], find the value of S when n = 42, a = 50, d = −2.”

Can someone please explain, I got the answer from a friend, but still don’t get how to get there (answer is s=378)

User Karthik V
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1 Answer

4 votes

Answer:

see explanation

Explanation:

Note that the formula given is incorrect.

The formula for the sum to n terms of an arithmetic sequence is


S_(n) =
(n)/(2) [ 2a + (n - 1)d ]

Substituting the given values into the formula


S_(42) =
(42)/(2) [ (2 × 50) + ( - 2 × 41) ]

= 21 ( 100 - 82)

= 21 × 18 = 378

User Abdi Hamid
by
7.5k points

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