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## Answers

nth term is 4n-1

then

if n=1 ; 4(1)-1 = 3

n= 2 ; 4(2)-1 = 7

n= 3 ; 4(3)-1 = 11

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their common difference is 4

so that these are in the A.p.

3,7,11,...........................

the sum of nth term formula is

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

the sum of first 20 terms are

= 20/2[2(3)+(20-1)4]

= 10[6+(19)4]

= 10×[6+76]

= 10(82)

= 820.

the sum of the first 20 terms is 820.