Question

in an AP 8f a=1 , An=20 , and Sn= 420 then find the value of n

Answer 1

Answer:

40

Step-by-step explanation:

Sn = n/2 (a + An)

420 = n/2 ( 1 + 20)

420 * 2 = 21n

n = 840/21

n = 40

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Answer 2

♡♡Hey. Here is your answer♡♡

》》Given In AP

a=1

An=20

Sn=420

Using formula for sum of AP

▪︎Sn= n/2 × (a+l)

=> 420 = n/2 × (1+20) {bcoz An=l}

=> 420×2 = 21n

=> 840= 21n

=> n= 840/21

=> 40

Therefore n=40

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