As a reminder, an arithmetic sequence or arithmetic progression (AP) is a progression in which the difference (called the tolerance) between two consecutive terms in the progression is constant.
Therefore, the expression for the nth term is
an = a₁ + (n − 1) d
and in general
an = am + (n - m) d.
where d is the difference.
For an arithmetic series with initial term a₁ and difference between terms of d
an = a₁ + (n − 1) d
We are told
d = -2 and
S8 = 240
So,
a1 = 240 = a1 + ( 30 - 1) * ( -2 ) = a1 - 58
a1 = 240+58 =298
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