# What is the sum of first n terms of the AP a, 3a, 5a, ....

Here, first term = a

Common difference = 3a - a = 2a

Now, Sum of first n terms of an AP is given by:

Sn = [2a + (n - 1)d]

where a is the first term and d is the common difference.

Sum of first n terms of given AP is given by:

Sn = [2a + (n - 1)2a]

= [2a + 2an - 2a]

= [2an]

= n2a

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