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)(xa)y=f(a)+f(a)(xa).
 

This is simply derived from the point-slope form of the equation of a line yy1=m(xx1)yy1=m(xx1) 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=1214(x2)y=1214(x2)
This figure has two parts a and b. In figure a, the line f(x) = 1/x is shown with its tangent line at x = 2. In figure b, the area near the tangent point is blown up to show how good of an approximation the tangent is near 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=1214(2.12)=0.475y=1214(2.12)=0.475

The actual value of f(2.1)f(2.1) is given by

f(2.1)=12.10.47619f(2.1)=12.10.47619

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=1214(102)=122=1.5y=1214(102)=122=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)(xa)f(x)f(a)+f(a)(xa) for xx near aa

We call the linear function

L(x)=f(a)+f(a)(xa)L(x)=f(a)+f(a)(xa)

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 9.19.1.

Find the linear approximation of f(x)=sinxf(x)=sinx at x=π3x=π3 and use it to approximate sin(62)sin(62).

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.