Implicit Differentiation and Related Rates

This section covers:

Introduction to Implicit Differentiation

Up to now, we’ve differentiated in explicit form, since, for example, y has been explicitly written as a function of x.

But sometimes, we can’t get an equation with a “y” only on one side; we may have multiply “y’s” in the equation.  In these cases, we have to differentiate “implicitly”, meaning that some “y’s” are “inside” the equation.  This is called implicit differentiation, and we actually have to use the chain rule to do this.   Here’s an example of an equation that we’d have to differentiate implicitly:   \(y=7{{x}^{2}}y-2{{y}^{2}}-\sqrt{{xy}}\).  Do you see how it’d be really difficult to get y alone on one side?

So the main thing to remember is when you are differentiating with respect to “x” and what you are differentiating only hasx’s” in it (or constants), you just get the derivative the normal way.  But if you are differentiating with respect to x, and what you are differentiating has another variable in it, like “y”, you have to multiply by  \(\frac{{dy}}{{dx}}\text{ (}{y}’)\):

Implicit Differientation Graphic

After we do the differentiation, we want to solve for the  \(\frac{{dy}}{{dx}}\)  by getting it to one side by itself (and we may have both x’s and y’s on the other side, which is fine).  So here are the steps for differentiating implicitly:

Implicit Differentiation

  1. Differentiate both sides of equation with respect to x. When are you differentiating variables other than x (such as “y”), remember to multiply that term by \(\frac{{dy}}{{dx}}\text{ (}{y}’)\)!
  2. Move to the left side of the equation and move all other terms to the right side (even if they have x’s and y’s in them).
  3. Factor out the on the left side of the equation, and solve for .

                                     Example:    \(\displaystyle \begin{array}{l}{{x}^{2}}+{{y}^{2}}=25\\2x+2y{y}’=0\\2y{y}’=-2x\\{y}’=-\frac{x}{y}\end{array}\)

 

I know it looks a bit scary, but it’s really not that bad!

Let’s do some non-trig problems first.  Do you see how we have to use the chain rule a lot more?  And note how the algebra can get really complicated!

Implicit Differientation Problems

Let’s try some problems with trig now.  Note that with trig functions, it’s hard to know where to stop simplifying, so there are several “correct” answers.

Implicit Differientation with Trig Functions

Equation of the Tangent Line with Implicit Differentiation

Here are some problems where you have to use implicit differentiation to find the derivative at a certain point, and the slope of the tangent line to the graph at a certain point.  The last problem asks to find the equation of the tangent line and normal line (the line perpendicular to the tangent line – take the negative reciprocal of the slope) at a certain point.

Note that we learned about finding the Equation of the Tangent Line in the Equation of the Tangent Line, Tangent Line Approximation, and Rates of Change section.

Slopes and Equations of Tangent Lines

Here’s a problem where we have to use implicit differentiation to find the points at which the graph of the equation has a horizontal and vertical tangent line.  Remember that for the horizontal tangent line, we set the numerator to 0, and for the vertical tangent line, we set the denominator to 0.

Points at Horizontal and Vertical Tangent LinesUsing Implicit Differentiation to Find Higher Order Derivatives

Here’s a problem where we have to use implicit differentiation twice to find the second derivative \(\displaystyle \frac{{{{d}^{2}}y}}{{d{{x}^{2}}}}\,\,\,or\,\,\,{y}”\).  This one is really tricky, since we need to substitute what we got for \(\frac{{dy}}{{dx}}\,\,or\,\,{y}’\) to simplify.

Note that in this example, we also substituted the original function back in to simplify further.  (The reason I substituted at the end is because the resulting answer was one of the choices on a multiple choice test question.  Your answer could be in a number of different forms.)

Implicit Differientation Twice

Related Rates

I used to have such a problem with related rates problems, until I began writing down the steps to do them.  And it helps to remember that the rates in these problems typically are differentiated with respect to time, or  \(\frac{{d\left( {\text{something}} \right)}}{{dt}}\).

Hope these steps help:

  1. Draw a picture and label any amounts that could be changing. These amounts will be variables, even if the problem gives you an amount at a certain time for these parts (the “when”).  In other words, understand what is changing and what isn’t.  I find it helpful to draw an arrow to show where the picture is “moving”.
  2. On the picture you’ve drawn, label places that are changing (you’ll need variables), and places that aren’t changing (constants). For constants, you can put numbers on the picture, if they are given.
  3. Write down exactly what the problem gives you, and what you need to solve for. For example, you may write down  “Find  \(\frac{{dA}}{{dt}}\)  when r = 6”.  Remember again that the rates (things that are changing) have “dt” (with respect to time) in them – pretty much always.  Also, determine what snapshot in time (or in another variable) the question is asking for (the “when”).
  4. For the rates, remember that rates with words like “filling up” means a positive volume rate, “emptying out” means a negative volume rate.
  5. Now we have to figure out a way to relate all the variables together. Write an equation that relates all of the given information and variables that you’ve written down.  A lot of times this is a geometry equation for volume, area, or perimeter.  Also, you’ll see the Pythagorean Equation or the trigonometric functions, such as SOH-CAH-TOA.  Remember that shapes stay in proportion, so we may have to use geometrically similar figures and set up proportions.
  6. Simplify by trying to put everything in as few variables as possible before differentiating (you may have to substitute some variables by solving in terms of other variables). For example, you may need to relate length and width together if you have a perimeter.  I find this the hardest part – how to know what to plug in before I differentiate, and what to plug in after.  The rule of thumb is that when you have values that say “when” something happens, these are put in after differentiating.
  7. Use implicit differentiation to differentiate with respect to time. Hopefully the rates  (\(\frac{{d\left( {\text{something}} \right)}}{{dt}}\))  you have left at this point are rates that either you have values for, or need values for.
  8. Fill in the equation with what you know (for example, the “when”s). Solve the equation for what you need (what you don’t know).
  9. Make sure you are answering the question that is being asked, and the units are correct. You’d hate to do all this work, and then answer the wrong question.

Here are some examples:

Related Rates 1

Here are more problems:

Related Rates Volume

Related Rates Lamppost

Here’s one that involves right triangle trigonometry:

Related Rates Trig

Understand these problems, and practice, practice, practice!


Click on Submit (the arrow to the right of the problem) to solve this problem. You can also type in more problems, or click on the 3 dots in the upper right hand corner to drill down for example problems.

If you click on “Tap to view steps”, you will go to the Mathway site, where you can register for the full version (steps included) of the software.  You can even get math worksheets.

You can also go to the Mathway site here, where you can register, or just use the software for free without the detailed solutions.  There is even a Mathway App for your mobile device.  Enjoy!

On to Curve Sketching – you’re ready! 

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