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### Course: Algebra (all content)>Unit 7

Lesson 13: Average rate of change word problems

# Average rate of change word problem: graph

Average rate of change tells us how much the function changed per a single time unit, over a specific interval. It has many real-world applications. In this video, we find the average rate of descent of a skydiver over a specific time interval.

## Want to join the conversation?

• How do you know to do 375 - 650, and not the reverse?
• It doesnt matter. If you chose 375-650 you should put its correspondig x coordinate values in the same order (8-3) and viceversa (650-375)/(3-8).

The signs do the work
• why we didn't take average of 700-200 ?
(200-700)/(10-0) = -50m/s
• Because we were asked the average rate of change between 3 and 8 seconds after she jumped, not between 0 and 10 seconds.
• At and , why does Sal approximate that h(3) is 650? We can clearly see that when x is 3, y is 650!
• If you look at the graph closely, you will see h(3) is actually slightly above 650. The graph is not crossing directly at y=650. So, it is an approximation.
• Why was H (8) used first in the formula? What are the rules in what goes in what order?
• It does seem confusing but after understanding it it's quite simple.
First, let's redo the calc.
1. change of H/change of Time
2. H(8) - H(3)/8 - 3
3. 375 - 650/5
4. -275/5
5. -55

And now let's do this the other way.
1. change of H/change of Time
2. H(3) - H(8)/3 - 8
3. 650 - 375/-5
4. 275/-5
5. -275/5 ( Hmmm... something seems similar )
6. -55

So basically no matter which one we put first we always get the same answer. This is one of the special things about maths. When we are calculating the rate of change of things no matter which value you put first you always get the same answer!

Hope this helped ( even if help came after 2 years )!
• There are two sections of the graph, x^2 and x. It's hard to believe Theresa slowed down to terminal velocity. I can imagine something speeding up to terminal velocity but not slowing down. What's going on?
• It's just the problem they use. It bugs me to, but sometimes you just have to ignore the setting of the question, and focus on the type and numbers.
• how would you do a line that goes up and down
• These "lines" represent data, or cause and effect information gathered about the world. To get a vertical line from an experiment would mean that when you test a single situation (x) that the response of the world is that everything and anything can happen (range is plus/minus infinity). Also, because there's only a single value of x, we are doing an experiment at only one data point, which is not an experiment. A vertical line cannot happen in real life and is not considered a "function". There is, however, a way to express a vertical line, but not in y=mx+b format. Just write an equation that says x never changes and y can be anything. For example, x=2.