If the 3rd and the 9th terms of an A.P. are 4 and − 8 respectively, then which term of this A.P is zero.
(REQUIRED QUALITY ANSWER)
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an=a+(n−1)d
a3=4=a+2d ....(1)
a9=−8=a+8d ....(2)
∴4−2d=−8d−8
−2d+8d=−8−4
6d=−12
d=−2
a=4−2d
=4+4=8
an=0,a=8,d=−2
0=8+(n−1)×(−2)
0=8−2n+2
2n=10
n=5
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