Question

in an a.p the first term is -5 and last term is 45 if the sum of all the term is 120 then n is equal to​

Answer 1

Answer:

Step-by-step explanation:

Given,

First term of an A.P = - 5

Last term of an A.P = 45

Sum of all terms in an A.P = 120

To Find:

Number of terms in A.P

Formula Required:-

$\sf S_n = \dfrac{n}{2}[a + l]$

Where,

n is number of terms

a is first term

l is last term.

Solution:-

According to question,

S_n = 120

a = - 5

l = 45

Substituting the values:-

$\sf 120 = \dfrac{n}{2}[-5 + 45]$

$\sf 120\times 2 = n[40]$

240 = n[40]

n = 240/40

n = 6

$\sf\therefore$ Number of terms in an A.P = 6