The sum of the third and the ninth term of an
a.P is 8. Find the sum of the first 11 terms of the progression
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Answer:
Tn=a+(n-1)d
Sn=n/2[2a+(n--1)d
A.T.Q.
Third term +ninth term=8
a+2d+a+8d=8
2a+10d=8...........(1)
.
2(a+5d)=8
a+5d=8/2=4
S11=11/2[2a+10d]
=11/2 ×[8]=(11/2 )×8=11×4=44 [using(1)]
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