The sum of the first 20 terms of the sequence a(n)=7n+2

A  =  7n + 2

a₁ = 7*1 + 2 = 9

a₂ = 7*2 + 2 = 16

a₃ = 7*3 + 2 = 23
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a₂₀ = 7*20 + 2 = 142

This is an Arithmetic progression, a = 9,   last term = 142

Sum of the 20 terms using:  n/2(a + l)      where l = last term.
 
 =  20/2(9 + 142)

= 10 * 151

= 1510

Sum of the first 20 terms = 1510