Question

In an ap the sum of first n terms is 3n2+5n and its kth term is 64

Answer 1

hope it is correct.
n=k

Answer 2

Answer:

Step-by-step explanation:

S = 3n2 + 5n

S1 = a1 = 3+5 = 8

S2 = a1 + a2 =6+10 =16                    ⇒a2 = 8

S3 = a1 + a2 + a3  = 9 + 15 = 24        ⇒a3 = 8

∴ AP is 8,8,8,.....

Thus a =8 , d=0          

Sm = 164

Sum of AP = n/2 [2a + (n-1)d]

164 = m/2 [2(8) + (m-1)0]

164 = m/2[16+0]

164 = m * 16 / 2

m = 164 * 2 /16

m = 20.5