a=-8
d=+2
n=?
S(n) = n/2[2a+(n-1)d]
90 = n/2[-16 + 2(n-1)]
this works out to
n^2 - 9n - 90 = 0
this factors nicely and has two answers, one of which will make no sense.
Let me know what you got.
how many terms of the series -8,-6-,-4.... make the sum 90?
3 answers
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