# In an AP, if a = 1, an = 20 and Sn = 399, then the what is the value of n?

Given:

First term, a = 1,

Nth term, an = 20,

Some of n terms, Sn = 299

To find: The value of n.

Formula Used:

Sum of “n” terms of an AP:

Where Sn is the sum of first n terms

n = no of terms

a = first term

l = last term

Explanation:

Using the formula

n = no of terms

l = an = last term

2Sn = n (a + an)

2(399) = n (1 + 20)

n = 38

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