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  • Find the sum to n terms of the A.P., whose k th term is 5k plus 1.

7.  Find the sum to n terms of the A.P., whose k^{th} term is 5k + 1.

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best_answer

Given : a_k=5k+1

\Rightarrow \, \, a+(k-1)d=5k+1

\Rightarrow \, \, a+kd-d=5k+1

Comparing LHS and RHS , we have

a-d=1        and     d=5

Putting value of d,

a=1+5=6

S_n=\frac{n}{2}[2a+(n-1)d]

S_n=\frac{n}{2}[2(6)+(n-1)5]

S_n=\frac{n}{2}[12+5n-5]

S_n=\frac{n}{2}[7+5n]

Posted by

seema garhwal

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