SOLUTION: 1) Given that for an AP seventh term=43 and second term=91,find the valves of a,d and n.(11). 2) The first term of an AP is -18 and the last term is 115.If the sum of the progr

Question 861041: 1)
Given that for an AP seventh term=43 and second term=91,find the valves of a,d and n.(11).
2)
The first term of an AP is -18 and the last term is 115.If the sum of the progression is 970.Find the number of terms in the AP and the common difference.
Answer by richwmiller(17219) (Show Source): You can put this solution on YOUR website!
Already answered the first one, although the answer isn't pretty.
S = �(a + an)n
S = �(2a + (n-1)d)n
an = a + (n - 1)d
970 = �(-18 + 115)n
970 = �(97)n
10=n/2
n=20 terms
133 = (19)d
d=7 difference
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