Question

The sum of first n terms of an ap is 3n^2+5n find first term and commondifference
${?}^{2}$
​

Answer 1

Step-by-step explanation:

Given, Sn = 3n^2 + 5n

to find, the value of a and d

sol. Sn = 3n^2 + 5n

S1 = 3(1)^2 + 5(1)

S1 = 8

Hence, S1 = T1 = a = 8

Now, S2 = 3(2)^2 +5(2)

S2 = 12 + 10

S2 = 22

Then, S2 = T1 + T2

22 = 8 + T2

T2 = 22 - 8

T2 = 14

since, d = T2 - T1

d = 14 - 8

d = 6

Hence, we get a=8 and d=6