you failed to include 12/2 in your formula of sum of 12 terms.

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you failed to include 12/2 in your formula of sum of 12 terms.

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[2(a₁) + (n-1)(d)]

n = 12 a₁= 1/5 d =-1/2

S₁₂ = 12/2 [2(1/5) + (12-1)(-1/2)]

S₁₂ = 6 [2/5 + (11)(-1/2)]

S₁₂ = 6 [2/5 - 11/2]

S₁₂ = 6 [

S₁₂ = 6 (-11/10)

S₁₂ = 3 (-11/5)