The formula for any geometric sequence is an = a1 · rn - 1, where an represents the value of the nth term, a1 represents the value of the first term, r represents the common ratio, and n represents the term number. What is the formula for the geometric sequence 1, -2, 4, -8, ...? an = 1 · (-2)n - 1 an = -2 · 1n - 1 an = -8 · (-2)n - 1 an = 1 · 2n - 1