Answer:

Step-by-step explanation:
To avoid confusion, distribute that negative amongst all terms in the second set of parentheses:

** The above answer is written in reverse, which is the exact same result.
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The formula<span> will be a</span>n<span> = a</span>12n - 1<span> or a</span>n<span> = (1)2</span>n - <span>1</span>