Question

a=21,d=(-3)and if an =(-81) We have to find n what is the value of n

Answer 1

Hey mate here is your answer.

We know that

an = a + (n - 1)d

(-81) = 21 + (n - 1)(-3)

- 81 = 21 - 3n + 3

- 81 - 21 = - 3n + 3

- 102 - 3 = - 3n

- 105 = - 3n

3n = 105

n = 105 / 3 = 35

Therefore the value of n is 35.

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Answer 2

QUESTION :

a=21,d=(-3)and if an =(-81) We have to find n what is the value of n

ANSWER :

here it is given that

in an AP

common difference= d = -3

also ,

nth term = an = -81

also

first term= a=21

we have to find that

no. of terms = n = ?

now ,

as we know that

nth term = first term+(no. of terms-1)× common difference

i.e. an= a+(n-1)d

so as

an = -81

we can write an as

a+(n-1)d = -81

(n-1)d=-81-a

but a=21

(n-1)d=-81-21

(n-1)=-102/d

but we know that d=-3

so (n-1)=-102/-3

=> (n-1)=34

=> n=34+1

=> n=35

NOTE :

while Solving such questions

you must know the formula required to solve the question