The sum of first 20 terms of an arithmetic sequence is1060 and its 5th term is 20. Find the 15th term of sequence.
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S²⁰ = 20/2 { 2a +19 d} = 10{ 2a +19 d}
=> 20a+190d =1060
=>2a+19d = 106 ........1
a⁵ = a+4d = 20 .........2
form eq2
a= 20-4d
put a for d
2a+19d = 106
2(20-4d) +19d = 106
40- 8d +19d =106
11d = 66
d= 6
put d for a
a = 20-4d = 20-4×6 = 20-24 = -4
hence the 15th term is a+14d = -4+14×6 = 80
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