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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

2 answers

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

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

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