a horizontal line; a melting ice cube.
a constant interval appears as a horizontal line. the graph in the interval has a slope m = 0.
here's an example of a constant interval.
you take an ice cube from the freezer and place it in a container on the counter.
the temperature of the ice cube is -5 °c. the ice cube will warm to 0 °c, and then it will start to melt. while it is melting, the temperature is constant at 0 °c.
when the ice has completely melted, the water will start warming to room temperature.
a plot of temperature vs. time would look something like the graph below.
the interval ab, where the ice is melting, is a constant interval.
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