Respuesta :

Let's calculate the sum:

\[ S_{12} = \frac{12}{2}[2(1) + (12-1)(-5)] \]

\[ S_{12} = 6[2 - 55] \]

\[ S_{12} = 6[-53] \]

\[ S_{12} = -318 \]

So, the sum of the first 12 terms of the given sequence is -318.

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