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In an arithmetic sequence the sum of the first nineterms is 279 and the sum of the first twenty terms is 1280.Then,

(a) what is the 5th term of the sequence ?

(b) what is the 16th term of the sequence?

(c) write the sequence .

a-4d, a-3d, a-2d, a-d, a, ....

9th term will be: a+4d

Sum of first 9 terms = 9a = 279

(a) 5th term is a = 279/9 = 31

(b) Sum of 20 terms = 9a + (a+5d)+(a+6d)......+(a+15d) = 1280

so, 110d = 1280-31*20 = 660

d = 660/110= 6

16th term = a+11d = 31+11*6=97

(c)Substitute a & d to get the sequence

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