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the sum of the first 21 term of the progression,-18, -15, -12,

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a = -18
d = 3
S21 = 21/2 (2a+20d) = ____

21/2 (2a+20d)
10.5 (2(-18)+20(3))
10.5[-36+60]
10.5×24=252😜😜

The sum of the first 21 terms of the progression

252

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