Question

Question 7 Find the sum to n terms of the A.P., whose kth term is 5k + 1.

Class X1 - Maths -Sequences and Series Page 185

Answer 1

$Given,\\kth\:term(T_k)=5K+1\\putting\:K=1,2,3,......\\T_1=5\times1+1=5+1=6\\T_2=5\times2+1=10+1=11\\T_3=5\times3+1=15+1=16\:and\:so\:on.\\\\a=6,d=11-6=5\\now,\\S_n=\frac{n}{2}[2a+(n-1)d]\\\\=\frac{n}{2}[2\times6+(n-1)5]\\\\=\frac{n}{2}[12+5n-5]\\\\=\frac{n}{2}[5n+7]$

Answer 2

Answer is in attachment.