If Pierre is going downhill, then the graph is decreasing:

So, our graph is decreasing on

Notice that I'm not using the endpoints on these:

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Think about it...  When Pierre is standing right ON x = -3 ...

he's not going uphill or downhill. He's just standing there!

What if Pierre is walking on

?

That line is horizontal (slope of 0). He's not going uphill or downhill, so the graph is not increasing or decreasing there.

TRY IT:

For the following graph, list the intervals where the graph is increasing and decreasing:

*Remember to answer in interval notation using only x values (no y values allowed)!