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# Worked example: positive & negative intervals

Finding the positive or negative intervals of a function from its graph. Created by Sal Khan.

Video transcript

A function, f of x,
is plotted below. Highlight an interval where
f of x is less than 0. So f of x-- which is
really being plotted on the vertical axis
right over here-- x is the horizontal axis. f of x being less
than 0 really means that the graph is
below the x-axis. So the function is negative in
this interval right over here and this interval over here. So I could put this
anywhere right over here, or I could stick it
anywhere right over here. Let me stick it right over here. There we go. Got it right. Let's do a couple more. So the function
is plotted below. Highlight an interval where
f of x is greater than 0. So I could do this
area right over here where the function
is above the x-axis, or I could do this
area right over here where the function goes
way above the x-axis. Well, it even goes off the page. So let's stick it
right over here. Let's do one more. So highlight an interval where
f of x is greater than 0. Once again, I can do this
region right over here where the function
is above the x-axis, or over here where
it's above the x-axis. I'll do it here just for fun. There we go.