Asked by Victoria

The sum of the first 21 terms of the progression,-18,-15,-12,is (solution)
Answered by mathhelper
a = -18
d = 3

sum(21) = (21/2)(2a + 20d)
= (21/2)(-36 + 60) = .....
Answered by Anonymous
looks like arithmetic sequence with a = -18 and d = +3
Tn = -18 + (n-1) * 3
You can look up the sum of the first 21 terms of an arithmetic sequence
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