Which formula would you use to find the total number of shaded triangles in the first 10 Sierpinski triangles?

The number of shaded triangles in each successive Sierpinski triangle is multiplied by 3, giving 1, 3, 9, and so on. The total for the first 10 triangles is the geometric sum 1 + 3 + 3² + ⋯ + 3⁹, which equals (1 − 3¹⁰)/(1 − 3) = (1 − 3¹⁰)/(−2). Therefore, choice A, (1 − 3¹⁰)/(−2), is the correct formula.

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