Question

In an A.P 1st term is 1 and last term is 20. Their sum is =399 then N= ?

Answer 1

Answer:

THERE ARE 38  TERMS

Step-by-step explanation:

Answer 2

Answer:

N = 38.

Step-by-step explanation:

Given:- In an A.P., 1st term = 1, last term = 20 and Sum of all terms = 399.

To Find:- Value of N i.e. number of terms in the A.P.

Solution:-

A.P. or Arithmetic progression is a series of number with a constant difference between the number.

Let's denote the first term as a, last term as L, common difference as cd and number of terms as N.

We know that,     L = a + (N - 1) × cd

                    Sum of all terms = N/2 [2a + (N - 1) × cd]

                                                = N/2 [a + L]

Substituting the values sum = 399, a = 1 and L = 20 in the above formula, we get

⇒ 399 = N/2 [ 1 + 20 ]

⇒ N/2 = 399 ÷ 21

⇒ N = (399 ÷ 21) × 2

⇒ N = 19 × 2

⇒ N = 38

Therefore, number of terms is 38.

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