Final answer:
The average rate of change of the function a(t) = 1 / (t + 3) on the interval [9, 9 + h] is -1 / (12 + h), but the options given are all positive and none match the correct expression. The provided options may be incorrect or based on a misunderstanding of the question.
Step-by-step explanation:
The student is asking to find the average rate of change of the function a(t) = 1 / (t + 3) on the interval [9, 9 + h]. The average rate of change is calculated as the difference in function values over the difference in t values, which translates to (a(9 + h) - a(9))/(9 + h - 9). Plugging the values into the function gives us:
a(9+h) = 1 / (9 + h + 3) = 1 / (12 + h)
a(9) = 1 / (9 + 3) = 1 / 12
Therefore, the average rate of change is:
((1 / (12 + h)) - (1 / 12)) / h = (1 / (12 + h) - 1 / 12) / h = (12 - (12 + h)) / (12(12 + h)h) = -h / (12(12 + h)h) = -1 / (12 + h)
Since the change is from a larger value to a smaller value, the average rate of change is negative. However, the options provided do not include a negative sign, implying there may be a misunderstanding in the options presented or the question asked. Assuming the absolute value is sought, the positive form would be 1 / (12 + h), which is not listed in the options provided.