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

Algebra ->  Sequences-and-series -> 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      Log On


   



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) About Me  (Show Source):
You can put this solution on YOUR website!
Already answered the first one, although the answer isn't pretty.
S = ½(a + an)n
S = ½(2a + (n-1)d)n
an = a + (n - 1)d
970 = ½(-18 + 115)n
970 = ½(97)n
10=n/2
n=20 terms
133 = (19)d
d=7 difference