# How can derivatives be used? What is their..purpose?

• Apr 17th 2009, 03:14 PM
Theoretical Physics '18
How can derivatives be used? What is their..purpose?
Hello all,
I am a 9th grade student in a college prep program, and I have started to learn Derivatives, and Integrals. (I am taking Algebra II Honors PIB and Geometry Honors PIB right now) I can derive very well (for a 9th grader with no formal instruction), but I only have a faint idea of how these derivatives are useful. How do are they used? Examples are appreciated (Nod)

Thank you!
-TP
• Apr 17th 2009, 03:19 PM
skeeter
look at Dr. Math's reply to a similar question ...

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• Apr 17th 2009, 03:26 PM
Theoretical Physics '18
Quote:

Originally Posted by skeeter
look at Dr. Math's reply to a similar question ...

Math Forum - Ask Dr. Math

yea, I saw that already. Although i found it useful, I wanted to give this a shot as well. (Talking)
• Apr 18th 2009, 05:05 PM
mr fantastic
Quote:

Originally Posted by Theoretical Physics '18
yea, I saw that already. Although i found it useful, I wanted to give this a shot as well. (Talking)

Everything that can be said on this subject will have been said many times elsewhere - use Google.
• Apr 18th 2009, 08:32 PM
VonNemo19
Maxima and Minima
I'll give you an example:

Let's say that a business must buy certain materials in order to keep operations going. Let's also say that the cost of buying these materials varies during different months of the year according to the function
C=x^2-2x +1. At which month would the cost of materials be at a minimum?

Hint: You're looking at x on the interval (0,12].

If you need help with this problem let me know.

Use the derivative to find the answer! Not the graph!(Wink)
• Apr 18th 2009, 09:19 PM
derfleurer
Quote:

Everything that can be said on this subject will have been said many times elsewhere - use Google.
So why even bother? =p

Anyway, a very common example is derivation of a position-time graph. Let's say we have some distance R that can be found using the equation 2t^3 - 3t^2 + 2. By finding the first and second derivatives, we can learn a lot more about the object travelling this path than that it moved 2 units (we'll just say meters) at 0 seconds.

R' = 6t^2 - 6t

This is dR/dt, our change in distance divided by change in time. You may recognize it as velocity (without getting technical). So, now we not only know how far the object has travelled after x seconds, but we also know how fast it was going at x seconds.

Now if we derive once more, we'll get d(dR/dt)/dt. Which you may recognize as acceleration.

R'' = 12t - 6

What does this tell us? It tells us that our acceleration is constantly changing. But we can also pull something else from this; maximum and minimum velocity. Tell me, how would you identify a maximum velocity? It'd be one of the times the object stops accelerating, right? Surely, if the object stops accelerating at a point, than after that point it must either go up, down, or stay the same.

So set R'' = 0. When does this occur? When t is 1/2, right? So the object's acceleration is 0 at 1/2 seconds. But, that doesn't tell us whether we're dealing with a min or max. How do we tell? Well, what's happening to the left of the point? What's happening to the right?

At 0, our R'' is -6. So our acceleration is going down. At 1, our R'' is 6, meaning it's going back up again. This tells us that our absolute minimum value for R' (dR/dt) occurs at 1/2 seconds.
• Apr 18th 2009, 09:59 PM
mollymcf2009
Quote:

Originally Posted by Theoretical Physics '18
Hello all,
I am a 9th grade student in a college prep program, and I have started to learn Derivatives, and Integrals. (I am taking Algebra II Honors PIB and Geometry Honors PIB right now) I can derive very well (for a 9th grader with no formal instruction), but I only have a faint idea of how these derivatives are useful. How do are they used? Examples are appreciated (Nod)

Thank you!
-TP

A derivative of a function measures how the function changes given different values of x.

For example, the derivative of a linear function y = mx + b, gives you the slope, which is the rate of change of the line

For a quadratic function, a parabola for example, its derivative gives you the equation of a tangent line to the parabola at a particular point.

The derivative of a position function (i.e. where an object is at a certain time) is velocity, the instantaneous rate of change of the object. Then, the derivative the velocity is acceleration, the change in velocity over time.

So, generally, a derivative can calculate rates of change.

Hope that helps! Derivatives are FUN!!!!! It will make your mathematical life MUCH easier! Good luck! (Wink)