Final answer:
A decreasing function over a given interval does not necessarily have to be negative over the same interval; it simply means the function's output is becoming smaller as the input increases. It could be always positive, or start positive and then become negative.
Step-by-step explanation:
When we say a function is “decreasing” over a given interval, it means that as we move from left to right along that interval on the x-axis, the value of the function (the y-value) gets smaller. Importantly, the term “decreasing” does not refer to the sign of the function's values, just the behavior of the function as x increases.
For example, a function can be decreasing and still be above the x-axis the entire time (always positive, steadily decreasing). Similarly, a function might start above the x-axis (initially positive) and cross it, becoming negative (becoming negative at the end).
A negative slope indicates a decreasing function, but does not necessarily mean the function is negative. Conversely, a positive slope indicates an increasing function. When the slope is zero, the function is neither increasing nor decreasing; it's constant over that interval.