First of all, you are using the formula for a geometric, yours is an arithmetic
Secondly, the sentence
"the 20th sum of an arithmetic progression is -7 and the sum of the first 20 terms is 620" makes no sense to me
what does "the 20th sum" mean ?
the 20th sum of an arithmetic progression is -7 and the sum of the first 20 terms is 620. find the 1st and 30th terms
i am looking at using S20 = a(1-rn)/l-r = 620. but i am reallys tuck and think i am very wrong. please help.
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