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Question

Find the general term of A.P if Sn=3n2+5n. Also find m if Tm=68

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Solution

Sn =n2(a1+(n1)d)

=n2((a1d)+nd)

=n2(d2)+(a1d2)n

on comparing

3=d2d=6 and

a1d2=5a1=16

Tm=16+(n1)6

Now Tm=68=16(m1)6
72=6(m1)
12=6(m1)
m=13 Answer

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