If the sum of first 11 terms of an AP is 121 and that of 17 terms is 289. Find the sum
of first n terms.
pls help me
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Answer:
Sn=n²
Step-by-step explanation:
S11=121
S17=289
Sn=n/2×2a+(n-1)×d
S11=11/2×2a+10d
a+5d=11 ⇒1 (by simplifing above equation)
and S17=n/2×2a +16d
a+8d=17⇒2
a=1 and d=2 by subracting the two equations 1 and 2
and substuting in formula
Sn=n/2×2a+{n-1}×d
Sn=n²
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