Question

If the sum of n term of an ap is sn=3n2+5n then find its common difference

Answer 1

if n=1 then Sn=3×1+5

S1=8 and S2=11

apply sn general formula solve the eq you get d

Answer 2

S_{1}  = 3(1)^{2} + 5(1)

S_{1}  = 3+5=8

Now S_{1} = a_{1} = 8

S_{2} = 3(2)^{2} + 5(2)

S_{2}= 12 + 10 = 22

S_{2} = a_{1} +a_{2}

a_{2} = S_{2} - a_{1}a_{2}= 22 - 8 = 14

d = a_{2} -a_{1}

d=14-8\\Therefore, d=6