sum of first 10 terms of an arithemetic sequence is 320.sum of 15 terms is 705.what is the algebraic expresion?
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Step-by-step explanation:
s=n/2(2a+(n-1)d
in 1 st case of 10 terms n=10
s=10/2(2a+(10-1)d
5(2a+9d)
= 10a+45d =320. (1)
in second case sum of 15 terms is 705
s=15/2(2a+(15-1)d
15(a+7d)
15a + 105d=705. (2)
on solving 1 and 2 we get
a=5. d=6
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