Answer
The function is always decreasing.
Work Step by Step
Taking the derivative of the function, we get $-\frac{1}{3}$, so the function is always decreasing, so the statement is proven.
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.