If the sum of first 7 terms of an Ap is 49 and that of first 17 terms is 289, find the sum of its first n terms
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8
Answer:
answer n2. ( n Square)
Explanation:
S7=49
7/2(2a+6d)=49
2a+6d=49*2/7
2a+6d=14
a+3d =7__________1
S17=289
17/2(2a+16d)=289
2a+16d=289*2/17
2a+16d=34
a+8d=17__________2
solving equation 1&2
a+8d=17
a+3d=7
________
5d=10
d=2 substituting value of d in eq 1
a+3*2=7
a=7-6=1
sum of first n terms =n/2(2a+n-1*d)
=n/2(2+2n-2)
= n/2*2(1+n-1)
=n(1+n-1)
= n*n
SN =n2
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