Question

please answer the question

Answer 1

$\huge{\mathfrak{\red{\underline{Answer :-}}}}$

Given :-

First term(a) = 1

$\sf{a_{n} = 20}$

$\sf{S_{n} = 399}$

To Find :-

Value of n.

Solution :-

We know that,

$\huge{\boxed{\red{\bf{\underline{S_{n} = \frac{n}{2}(a +l)}}}}}$

_______________[Put Values]

399 = n (20 + 1)/2

399 = 21n/ 2

399 * 2 = 21n

798 = 21n

798/21 = n

38 = n

$\huge{\boxed{\red{\bf{\underline{n = 38}}}}}$