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math is fun arithmetic sequence
The first and last term of an ap are 21 And -47 Respectively if the sum of the series is given as -234 Calculate the number of terms in the ap find the common difference and the sum of the first 81terms
2 answers
Sum(n) = (n/2)(2a + (n-1)d) OR (n/2)(first + last)
let's use the second version
-234 = (n/2)(21-47)
n = 9
we already know a = 21
term(9) = a+8d = -47
21 + 8d = -47
d = -17/2
using the first sum formula this time
sum(81) = (81/2)(42 + 80(-17/2)) = -25839
let's use the second version
-234 = (n/2)(21-47)
n = 9
we already know a = 21
term(9) = a+8d = -47
21 + 8d = -47
d = -17/2
using the first sum formula this time
sum(81) = (81/2)(42 + 80(-17/2)) = -25839
3 answers