Calculus

Differentiation of Functions

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Differential of a Function

Solved Problems

Example 9.

Find the differential of the function

Solution.

We calculate the derivative using the product and chain rule:

The differential of the function is written as

Example 10.

Find the differential of the function

Solution.

First we determine the derivative using the chain rule:

Apply the double angle identity

Hence

The differential of a function is written in the form

Then

Example 11.

Find the differential of at the point when

Solution.

Substitute the values of and and calculate the differential

Example 12.

Find the differential of the function at

Solution.

Differentiate the given function:

At the point the derivative is equal to

Hence, the differential of the function at this point is

Example 13.

Use differential to approximate the change in as changes from to

Solution.

We calculate the differential of the function by the formula

The derivative is given by

At the initial point the derivative is equal to

Calculate the differential of the independent variable:

Then the differential is

The approximate value of the function at is equal to

Example 14.

Calculate the increment and differential of the function at the point when

Solution.

First we compute the increment of the function:

At the same values of and , the differential of the function is equal to

Example 15.

Find the differential of the function at

Solution.

Take the derivative of the function:

At the point we have

Then

Example 16.

Calculate the differential of the function at the point when

Solution.

Take the derivative:

At the derivative is equal to

So the differential at this point is

Example 17.

Find the differential of the function where and are differentiable functions of the variable

Solution.

Using the differentiation rules, we obtain:

Example 18.

Find the differential of the function where and are differentiable functions of

Solution.

where

Example 19.

The function is given implicitly by the equation Find its differential at the point

Determine the derivative of the implicit function. Differentiating both sides with respect to we obtain:

At the point , the derivative is equal

The differential at this point is respectively written as

Example 20.

The function is defined by the implicit equation Find its differential at the point

Solution.

We differentiate both sides of the equation with respect to and find the derivative

Calculate the value of the derivative at the given point

The differential of the function at this point is written in the following form:

Example 21.

Suppose that a force is determined by the law The distance is measured as Find the approximate value of the force

Solution.

Note that

We evaluate the differential at the point when As

and

then

Similarly we can find that for Therefore, the force is estimated to be in the range

Example 22.

A body is moving through a liquid. The resistance force depends on the speed of the body as where is the speed and is a constant. The speed was measured as Determine the approximate value of the resistance force if

Solution.

It is easy to see that

We estimate the approximate value of the force using the differential:

Take the derivative:

so when the derivative is equal to

Assuming we have

Similarly, we can find the differential for

Thus, the resistance force takes on values in the interval

Example 23.

The function is defined by the parametric equations Find the differential of the function at the point

Solution.

We calculate the corresponding values of the parameter from the equation

Make sure that the value satisfies the condition

When the derivative has the following value:

Thus, the differential of the function at the point is expressed by the formula

Example 24.

The function is defined by the parametric equations Find the differential of the function at the point

Solution.

First we determine the value of the parameter , which corresponds to the point It follows from the second equation that

Check the value of at

Find the derivative of the parametric function:

When the derivative is respectively equal to

Consequently, the differential of the function at the point has the form:

Example 25.

Given the composite function Express the differential of in an invariant form.

Solution.

We write the differential of the "outer" function:

Similarly, we find the differential of the "inner" function:

Substituting the expression for in the previous formula, we obtain the differential in invariant form:

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