Question

find the sum of first 20 terms of an AP 5,13,21​

Answer 1

Answer:

is 6440

Step-by-step explanation:

for sum of an A.P we have

S=n/2{2a+(n-1)d}

Here s=6440 , a=5 ,d=8 and we have to find n

Now

6440=n/2 {(2×5)+(n-1)×8}

6440=n/2 {10+(n-1)×8}

6440×2=n {10+8n-8}

6440×2=n(2+8n)

6440×2=2n(1+4n)

6440=n+4n^2

4n^2 +n-6440=0

4n^2 +161n -160n -6440=0

n(4n +161) -40(4n+161)=0

(n-40) (4n+161) =0

n=40 and n =-161/4 (discarded)

So sum to 40 terms of given A.P is 6440

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