Linear Approximations and Differentials: Learn It 1
Explain and use linearization to approximate a function’s value near a specific point
Calculate and interpret differentials to estimate small changes in function values
Measure the accuracy of approximations made with differentials by calculating relative and percentage errors
Linear Approximation of a Function at a Point
We have just seen how derivatives allow us to compare related quantities that are changing over time. In this section, we examine another application of derivatives: the ability to approximate functions locally by linear functions. Linear functions are the easiest functions with which to work, so they provide a useful tool for approximating function values.
Recall that the tangent line to the graph of ff at aa is given by the equation
y=f(a)+f′(a)(x−a)y=f(a)+f′(a)(x−a).
This is simply derived from the point-slope form of the equation of a line y−y1=m(x−x1)y−y1=m(x−x1) by adding y1y1 to both sides!
Consider the function f(x)=1xf(x)=1x at a=2a=2. Since ff is differentiable at x=2x=2 and f′(x)=−1x2f′(x)=−1x2, we see that f′(2)=−14f′(2)=−14.
Therefore, the tangent line to the graph of ff at a=2a=2 is given by the equation
y=12−14(x−2)y=12−14(x−2)
Figure 1. (a) The tangent line to f(x)=1xf(x)=1x at x=2x=2 provides a good approximation to ff for xx near 2. (b) At x=2.1x=2.1, the value of yy on the tangent line to f(x)=1xf(x)=1x is 0.475. The actual value of f(2.1)f(2.1) is 12.112.1, which is approximately 0.47619.
Figure 1a shows a graph of f(x)=1xf(x)=1x along with the tangent line to ff at x=2x=2. Note that for xx near 22, the graph of the tangent line is close to the graph of ff. As a result, we can use the equation of the tangent line to approximate f(x)f(x) for xx near 22.
If x=2.1x=2.1, the yy value of the corresponding point on the tangent line is
y=12−14(2.1−2)=0.475y=12−14(2.1−2)=0.475
The actual value of f(2.1)f(2.1) is given by
f(2.1)=12.1≈0.47619f(2.1)=12.1≈0.47619
Therefore, the tangent line gives us a fairly good approximation of f(2.1)f(2.1) (Figure 1b).
However, note that for values of xx far from 22, the equation of the tangent line does not give us a good approximation. If x=10x=10, the yy-value of the corresponding point on the tangent line is
y=12−14(10−2)=12−2=−1.5y=12−14(10−2)=12−2=−1.5,
whereas the value of the function at x=10x=10 is f(10)=0.1f(10)=0.1.
In general, for a differentiable function ff, the equation of the tangent line to ff at x=ax=a can be used to approximate f(x)f(x) for xx near aa. Therefore, we can write
f(x)≈f(a)+f′(a)(x−a)f(x)≈f(a)+f′(a)(x−a) for xx near aa
We call the linear function
L(x)=f(a)+f′(a)(x−a)L(x)=f(a)+f′(a)(x−a)
the linear approximation, or tangent line approximation, of ff at x=ax=a. This function LL is also known as the linearization of ff at x=ax=a.
linear approximation
Linear approximation, or tangent line approximation, is a mathematical method that uses the tangent at a specific point to estimate the values of a function near that point.
Find the linear approximation of f(x)=√xf(x)=√x at x=9x=9 and use the approximation to estimate √9.1√9.1.
Since we are looking for the linear approximation at x=9x=9, using the tangent line approximation, we know the linear approximation is given by
Figure 2. The local linear approximation to f(x)=√xf(x)=√x at x=9x=9 provides an approximation to ff for xx near 9.
Analysis
Using a calculator, the value of √9.1√9.1 to four decimal places is 3.0166. The value given by the linear approximation, 3.0167, is very close to the value obtained with a calculator, so it appears that using this linear approximation is a good way to estimate √x√x, at least for xx near 9. At the same time, it may seem odd to use a linear approximation when we can just push a few buttons on a calculator to evaluate √9.1√9.1. However, how does the calculator evaluate √9.1√9.1? The calculator uses an approximation! In fact, calculators and computers use approximations all the time to evaluate mathematical expressions; they just use higher-degree approximations.
Watch the following video to see the worked solution to this example.
For closed captioning, open the video on its original page by clicking the Youtube logo in the lower right-hand corner of the video display. In YouTube, the video will begin at the same starting point as this clip, but will continue playing until the very end.
Find the linear approximation of f(x)=sinxf(x)=sinx at x=π3x=π3 and use it to approximate sin(62∘)sin(62∘).
First we note that since π3π3 rad is equivalent to 60∘60∘, using the linear approximation at x=π/3x=π/3 seems reasonable. The linear approximation is given by
Therefore, the linear approximation of ff at x=π/3x=π/3 is given by Figure 3.
L(x)=√32+12(x−π3)L(x)=√32+12(x−π3)
To estimate sin(62∘)sin(62∘) using LL, we must first convert 62∘62∘ to radians. We have 62∘=62π18062∘=62π180 radians, so the estimate for sin(62∘)sin(62∘) is given by
Figure 3. The linear approximation to f(x)=sinxf(x)=sinx at x=π3x=π3 provides an approximation to sinxsinx for xx near π3.π3.
Watch the following video to see the worked solution to this example.
For closed captioning, open the video on its original page by clicking the Youtube logo in the lower right-hand corner of the video display. In YouTube, the video will begin at the same starting point as this clip, but will continue playing until the very end.
Linear approximations may be used in estimating roots and powers. In the next example, we find the linear approximation for f(x)=(1+x)nf(x)=(1+x)n at x=0x=0, which can be used to estimate roots and powers for real numbers near 11. The same idea can be extended to a function of the form f(x)=(m+x)nf(x)=(m+x)n to estimate roots and powers near a different number mm.
Find the linear approximation of f(x)=(1+x)nf(x)=(1+x)n at x=0x=0. Use this approximation to estimate (1.01)3(1.01)3.
Figure 4. (a) The linear approximation of f(x)f(x) at x=0x=0 is L(x)L(x). (b) The actual value of 1.0131.013 is 1.030301. The linear approximation of f(x)f(x) at x=0x=0 estimates 1.0131.013 to be 1.03.