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The sum of n terms of an AP is given by S = n*(T1 + Tn)/2.
Here, the kth term is 1 + 5k.
The first term is T1 = 1 + 5 = 6, the nth term is Tn = 1 + 5n.
Substitute in the expression for S
=> (n/2)(6 + 1 + 5n)
=> (n/2)(7 + 5n)
=> 7n/2 + 5n^2/2
The sum to n terms of the AP whose kth term is 1 + 5k is 7n/2 + 5n^2/2
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