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given a_{n =} a + nb
so a₁ = a+ b
a₂= a + 2b
a_{3 =} a + 3b

because a₂ - a₁ = a₃ - a₂ = b
so this is an A.P.

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so 1st term = a+ b
d = b

S_n = (n/2)( 2 ( a+b) + (n-1)b)
S_{20 =} (20/2)( (2a + 2b ) + 19b
= 10 (2a + 2b + 19b )
= 10 ( 2a + 21b)
S₂₀ = 20 a + 210 b