the 15th term of an arthemetic sequence is 40. find the 14th and 16th terms.find the sum of first 29 terms.
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Answer:
15th term: tn=a+(n-1)d
t15=a+(15-1)d
40-14d = a ➡ Equation (1)
Sn=n/2(2a+(n-1)d)
S29= 29/2(2a+(29-1)d)
=29/2(2a+28d)
=29/2(2*40-14d+28d) ➡ From 1
=29/2(80+14d)
=29/2*80+14d
= 29*40+14d
-14d= 1160
d=1160/-14
d=82
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