Question

what is the sum of first 20 terms of A.P 1, 21, 41,...?
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Answer 1

Answer:

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Step-by-step explanation:

1, 21 , 41 ,......?

now ,

a1 = 1

D = a2 - a1 = 21-1 = 20

n = 20

Sn= n/2 [2a + (n-1)d]

= 20/2 [2(1) + (20-1)(20) ]

= 10 ( 2 +400-20)

= 10(2+380)

= 10× 382

Sn = 3820

thus, the sum of first 20 terms is 3820.

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