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## Algebra 1

### Course: Algebra 1>Unit 8

Lesson 9: Intervals where a function is positive, negative, increasing, or decreasing

# 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.