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Find the sum of n terms of an A.P. whose...

Find the sum of `n` terms of an A.P. whose kth terms is `5k+1.`

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As, it is given that `k^th` term of the A.P. is `5k + 1.`
therefore, ` ak = a + (k – 1)d `
`=> a + (k – 1)d = 5k + 1 `
`=> a + kd – d = 5k + 1 `
...
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