Final answer:
The formula for the geometric sequence that begins with 11, -33, 99 is an = 11 * (-3)n-1.
Step-by-step explanation:
To find a formula for a geometric sequence, we need to find the common ratio.
The common ratio is found by dividing any term in the sequence by its previous term.
In this case, the common ratio is found by dividing -33 by 11, which is -3. So, the formula for the geometric sequence is:
an = a1 * rn-1
Here, a1 is the first term, which is 11, and r is the common ratio, which is -3. So, the formula for the given geometric sequence is:
an = 11 * (-3)n-1