The sum of first n terms of an ap is 3n^2-4n, find its 25th term
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Step-by-step explanation:
Sn=3n²-4n
n/2{2a+(n-1)d}=n(3n-4)
1/2{2a+(n-1)d}=(3n-4)
2a+(n-1)d=6n-8
2a+nd-d=6n-8
(2a-d)+nd=6n-8
by seperating
nd=6n & (2a-d)= -8
d=6 & 2a-6= -8
a= -1
tn=a+(n-1)d
t25=a+(25-2)d
= -1+24×6
= -1+144
=143
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