Combining Differentiation Rules

Learning Outcomes

  • Combine the differentiation rules to find the derivative of a polynomial or rational function

As we have seen throughout the examples in this section, it seldom happens that we are called on to apply just one differentiation rule to find the derivative of a given function. At this point, by combining the differentiation rules, we may find the derivatives of any polynomial or rational function. Later on we will encounter more complex combinations of differentiation rules. A good rule of thumb to use when applying several rules is to apply the rules in reverse of the order in which we would evaluate the function.

Example: Combining Differentiation Rules

For [latex]k(x)=3h(x)+x^2g(x)[/latex], find [latex]k^{\prime}(x)[/latex].

Example: Extending the Product Rule

For [latex]k(x)=f(x)g(x)h(x)[/latex], express [latex]k^{\prime}(x)[/latex] in terms of [latex]f(x), \, g(x), \, h(x)[/latex], and their derivatives.

Watch the following video to see the worked solution to Example: Extending the Product Rule.

Example: Combining the Quotient Rule and the Product Rule

For [latex]h(x)=\large \frac{2x^3k(x)}{3x+2}[/latex], find [latex]h^{\prime}(x)[/latex].

Try It

Find [latex]\frac{d}{dx}(3f(x)-2g(x))[/latex].

Example: Determining Where a Function Has a Horizontal Tangent

Determine the values of [latex]x[/latex] for which [latex]f(x)=x^3-7x^2+8x+1[/latex] has a horizontal tangent line.

Watch the following video to see the worked solution to Example: Determining Where a Function Has a Horizontal Tangent.

Example: Finding a Velocity

The position of an object on a coordinate axis at time [latex]t[/latex] is given by [latex]s(t)=\dfrac{t}{t^2+1}[/latex]. What is the initial velocity of the object?

Try It

Find the value(s) of [latex]x[/latex] for which the line tangent to the graph of [latex]f(x)=4x^2-3x+2[/latex] is parallel to the line [latex]y=2x+3[/latex].

Try It

Activity: Racetrack Safety at the Formula One Grandstand

Formula One car races can be very exciting to watch and attract a lot of spectators. Formula One track designers have to ensure sufficient grandstand space is available around the track to accommodate these viewers. However, car racing can be dangerous, and safety considerations are paramount. The grandstands must be placed where spectators will not be in danger should a driver lose control of a car (Figure 3).

A photo of a grandstand next to a straightaway of a race track.

Figure 3. The grandstand next to a straightaway of the Circuit de Barcelona-Catalunya race track, located where the spectators are not in danger.

 

Safety is especially a concern on turns. If a driver does not slow down enough before entering the turn, the car may slide off the racetrack. Normally, this just results in a wider turn, which slows the driver down. But if the driver loses control completely, the car may fly off the track entirely, on a path tangent to the curve of the racetrack.

Suppose you are designing a new Formula One track. One section of the track can be modeled by the function [latex]f(x)=x^3+3x^2+x[/latex] (Figure 4). The current plan calls for grandstands to be built along the first straightaway and around a portion of the first curve. The plans call for the front corner of the grandstand to be located at the point [latex](-1.9,2.8)[/latex]. We want to determine whether this location puts the spectators in danger if a driver loses control of the car.

This figure has two parts labeled a and b. Figure a shows the graph of f(x) = x3 + 3x2 + x. Figure b shows the same graph but this time with two boxes on it. The first box appears along the left-hand side of the graph straddling the x-axis roughly parallel to f(x). The second box appears a little higher, also roughly parallel to f(x), with its front corner located at (−1.9, 2.8). Note that this corner is roughly in line with the direct path of the track before it started to turn.

Figure 4. (a) One section of the racetrack can be modeled by the function [latex]f(x)=x^3+3x^2+x[/latex]. (b) The front corner of the grandstand is located at [latex](-1.9,2.8)[/latex].

  1. Physicists have determined that drivers are most likely to lose control of their cars as they are coming into a turn, at the point where the slope of the tangent line is 1. Find the [latex](x,y)[/latex] coordinates of this point near the turn.
  2. Find the equation of the tangent line to the curve at this point.
  3. To determine whether the spectators are in danger in this scenario, find the [latex]x[/latex]-coordinate of the point where the tangent line crosses the line [latex]y=2.8[/latex]. Is this point safely to the right of the grandstand? Or are the spectators in danger?
  4. What if a driver loses control earlier than the physicists project? Suppose a driver loses control at the point [latex](-2.5,0.625)[/latex]. What is the slope of the tangent line at this point?
  5. If a driver loses control as described in part 4, are the spectators safe?
  6. Should you proceed with the current design for the grandstand, or should the grandstands be moved?