The first term of an AP is $5$ , the last term is $45$ and the sum is $400$ . Find the number of terms.

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Solution

Formula,

$S_{l} = \frac{n}{2} [a + l]$

Given,

$400 = \frac{n}{2} [5 + 45]$

$400 = n [25]$

$∴ n = 16$

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