“if s = -- [2a + (n − 1)d], find the value of s when n = 42, a = 50, d = −2.”
can someone explain, i got the answer from a friend, but still don’t get how to get there (answer is s=378)

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

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

Substituting the given values into the formula

= [ (2 × 50) + ( - 2 × 41) ]

                          = 21 ( 100 - 82)

                          = 21 × 18 = 378

3,000,540

step-by-step explanation: