Q. 105.0( 8 Votes )

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

Answer :

**Given:**

First term, a = 1,

Nth term, a_{n} = 20,

Some of n terms, S_{n} = 299

**To find:** The value of n.

**Formula Used:**

Sum of “n” terms of an AP:

Where S_{n} 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 = a_{n} = last term

2S_{n} = n (a + a_{n})

2(399) = n (1 + 20)

n = 38

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