Asked by Ellie

For an arithmetic sequence, a_{19} = 78. If the common difference is -3, find
a_1 = (i found it to be 132)
the sum of the first 57 terms =??

-I know there is a formula but i cant think of what it is
Answered by Reiny
Sum<sub>n</sub> = (n/2)[2a + n-1)d]
Answered by Reiny
Sum<sub>n</sub> = (n/2)[2a + (n-1)d]
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