Step-by-step explanation:
(-infinity, -5)
(-1, +2)
(+4, +6)
(+6, +infinity)
for the infinity interval ends we have to use round brackets, because infinity is not a value, so it can never be reached.
for the others I have used round brackets, because formally, at exactly these points, the graph makes a turn of direction and actually has a flat tangent. a flat, horizontal tangent (a line) means its slope is 0. so the function or graph is actually not decreasing at that point.
but now it is essential what you teacher told you.
if he/she uses [ or ] in these cases, then please put them at all interval ends with specific numbers.
and then the last 2 intervals would be combined to
[+4, +infinity).
but normally, the original list above should be correct.
at -5, -1, +2, +4 and +6 there is a tangent with slope 0, which ends or interrupts the periods of decreasing.
