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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)

Respuesta :

Answer:

see explanation

Step-by-step explanation:

Note that the formula given is incorrect.

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

[tex]S_{n}[/tex] = [tex]\frac{n}{2}[/tex] [ 2a + (n - 1)d ]

Substituting the given values into the formula

[tex]S_{42}[/tex] = [tex]\frac{42}{2}[/tex] [ (2 × 50) + ( - 2 × 41) ]

                          = 21 ( 100 - 82)

                          = 21 × 18 = 378

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