Question

in an ap a=1 an=20 sn=399 find n

Answer 1

Answer:

The value of n is 38 in your question.

Answer 2

ANSWER:

• The value of n = 38

GIVEN:

• First term = 1

• Last term = 20

• Sum of n terms = 399

TO FIND:

• The value of n.

EXPLANATION:

$\boxed{\boxed{\large{\bold{S_n=\dfrac{n}{2}\bigg(2a + (n-1)d\bigg)}}}}$

$\boxed{\boxed{\large{\bold{A_n = a + (n-1)d}}}}$

$\boxed{\boxed{\large{\bold{S_n=\dfrac{n}{2}(a +A_n) }}}}$

$S_n = 399$

$a = 1$

$A_n = 20$

$399= \dfrac{n}{2}\bigg(1 + 20\bigg)$

$798= 21n$

$n = 38$

HENCE THE VALUE OF n = 38