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derivatives and application of derivatives

Module - 2 Calculus

Lesson - 2 Derivatives and Applications of Derivatives

**Overview**

Derivatives are an important concept in calculus that allow us to find the rate of change of a function at any given point. They have a wide range of applications in fields such as physics, engineering, and finance. Some common applications of derivatives include optimization problems, related rates problems, critical points, inflection points, concavity, and curve sketching. Understanding derivatives and their applications is crucial for solving real-world problems and making informed decisions in various fields.

**Introduction to derivatives: definition, notation, and essential concepts**

Derivatives are principal concepts in calculus that represent the rate of alter of a function at a given point. The derivative of a function f(x) at a point x=a is given by f'(a) and is characterized as the limit of the difference quotient as the distinction between x and a approaches zero:

```
f'(a) = lim [(f(x) - f(a))/(x - a)] as x approaches a
```

In other words, the derivative of a function represents the slant of the tangent line to the work at a given point.

For illustration, consider the function f(x) = x^2. The derivative of f(x) at any point x is given by:

```
f'(x) = lim [(f(x+h) - f(x))/h] as h approaches
= lim [((x+h)^2 - x^2)/h] as h approaches
= lim [(x^2 + 2xh + h^2 - x^2)/h] as h approaches
= lim [(2x + h)] as h approaches
= 2x
```

Hence, the derivative of f(x) = x^2 is f'(x) = 2x.

**Procedures for finding derivatives**

There are a few strategies for finding derivatives, including the power rule, product rule, quotient rule, chain rule, and others. These rules permit us to discover the derivative of more complex capacities by breaking them down into less complex components and applying a set of rules.

The power rule states that the subsidiary of a function f(x) = x^n is given by:

```
f'(x) = n*x^(n-1)
```

For case, the derivative of f(x) = x^3 is:

```
f'(x) = 3x^2
```

The product rule states that the derivative of a product of two functions f(x) and g(x) is given by:

```
(fg)'(x) = f'(x)g(x) + f(x)g'(x)
```

For example, consider the product f(x) = x^2 * sin(x). The derivative of f(x) is:

```
f'(x) = (2xsin(x)) + (x^2cos(x))
```

The quotient rule states that the derivative of a remainder of two capacities f(x) and g(x) is given by:

```
(f/g)'(x) = [f'(x)g(x) - f(x)g'(x)]/g(x)^2
```

For illustration, consider the function f(x) = x^2 / cos(x). The derivative of f(x) is:

```
f'(x) = [2xcos(x) + x^2sin(x)]/cos(x)^2
```

The chain rule allows us to find the derivative of composite functions, where one function is applied to the output of another function. The chain rule states that if y = f(g(x)), then:

```
dy/dx = (dy/dg)*(dg/dx)
```

For case, consider the work f(x) = (sin(x))^2. The subordinate of f(x) is:

```
f'(x) = 2*sin(x)*cos(x)
```

**Higher-order derivatives and their applications**

Higher-order derivatives are derivatives of derivatives. For example, the second derivative of a function is the derivative of its first derivative, and the third derivative is the derivative of its second derivative. Higher-order derivatives can provide information about the curvature, rate of change, and other properties of a function.

One application of higher-order derivatives is in optimization problems. For example, if we want to find the maximum or minimum value of a function, we can use the first and second derivatives to determine whether a critical point is a maximum, minimum, or saddle point. If the second derivative is positive at a critical point, the function has a local minimum at that point, while a negative second derivative indicates a local maximum. A critical point where the second derivative is zero is called an inflection point.

Another application of higher-order derivatives is in Taylor series expansions, which are used to approximate functions with polynomials. The Taylor series of a function at a point x=a is:

```
f(x) = f(a) + f'(a)(x-a) + (1/2!)f''(a)(x-a)^2 + (1/3!)f'''(a)(x-a)^3 + ...
```

where f'(a) is the first derivative of f(x) evaluated at x=a, f''(a) is the second derivative evaluated at x=a, and so on. The Taylor series can be used to approximate the value of a function near a given point, and higher-order derivatives can improve the accuracy of the approximation.

**Derivatives of trigonometric, logarithmic, and exponential functions**

Trigonometric functions such as sin(x) and cos(x) have subordinates that can be determined utilizing the chain rule:

```
d/dx sin(x) = cos(x)
d/dx cos(x) = -sin(x)
```

Logarithmic functions such as ln(x) have derivatives that can be derived using the chain rule and the fact that d/dx e^x = e^x:

```
d/dx ln(x) = 1/x
```

Exponential capacities such as e^x have a derivative that's rise to to the function itself:

```
d/dx e^x = e^x
```

Other functions can be derived using a combination of these rules and algebraic manipulation. For example, the derivative of f(x) = ln(sin(x)) can be found using the chain rule and the derivatives of sin(x) and ln(x):

```
d/dx ln(sin(x)) = (1/sin(x)) * cos(x)/sin(x) = cos(x)/sin(x)^2
```

**Implicit differentiation and related rates problems**

Implicit differentiation is a technique used to find the derivative of a function that is not expressed explicitly in terms of x. For example, consider the equation x^2 + y^2 = 25, which represents a circle with radius 5 centered at the origin. To find the derivative of y with respect to x, we can differentiate both sides of the equation with respect to x using the chain rule:

```
d/dx (x^2 + y^2) = d/dx (25)
d/dx (x^2 + y^2) = 2x(dx/dx) + 2y(dy/dx)
2x + 2y(dy/dx) = 0
dy/dx = -2x/2y
dy/dx = -x/y
```

Related rates problems involve finding the rate of change of one variable with respect to another, given the rates of change of other variables. For example, consider a ladder leaning against a wall. If the ladder is sliding down the wall at a rate of 1 meter per second, and the base of the ladder is sliding away from the wall at a rate of 2 meters per second, how fast is the top of the ladder sliding down the wall?

Let h be the height of the ladder and x be the distance from the wall to the base of the ladder. We are given that dh/dt = -1 (the negative sign indicates that the height is decreasing) and dx/dt = 2. We want to find the rate of change of the height of the ladder with respect to time, or dh/dt.

We can use the Pythagorean theorem to relate h and x:

```
h^2 + x^2 = L^2
```

where L is the length of the ladder. Differentiating both sides with regard to time utilizing the chain run the show gives:

```
2h(dh/dt) + 2x(dx/dt) = 2L(dL/dt)
```

We know that dx/dt = 2, and we can find dL/dt using the fact that the ladder is sliding down the wall at a rate of 1 meter per second. Since the ladder is always in contact with the wall, the vertical component of its velocity must be equal to the vertical component of the velocity of the point where it touches the wall. This means that dL/dt = dh/dt + 1.

Substituting these values into the equation above gives:

```
2h(dh/dt) + 2x(2) = 2L(dL/dt)
2h(dh/dt) + 4x = 2L(dh/dt + 1)
2h(dh/dt) + 4x = 2h(dh/dt) + 2
2x = 2
x = 1
```

Hence, we have h^2 + 1 = L^2, and dL/dt = dh/dt + 1. Substituting these values into the condition above gives:

```
2h(dh/dt) + 2(1)(2) = 2L(dh/dt + 1)
2h(dh/dt) + 4 = 2h(dh/dt) + 2(dh/dt + 1)
2 = 3(dh/dt)
dh/dt = 2/3 meters per second
```

This implies that the height of the step is diminishing at a rate of 2/3 meters per second.

In related rates problems, it is important to identify the variables that are changing with respect to time, and to use appropriate equations to relate these variables. Implicit differentiation can be used to find the derivatives of functions that are not expressed explicitly in terms of one variable, and can be useful in solving related rates problems.

**Applications of derivatives in optimization:**

1. One of the most important applications of derivatives is in optimization problems, where we seek to find the maximum or minimum values of a function. For example, a business might want to maximize its profits or a farmer might want to minimize the cost of producing crops.

To solve these problems, we use calculus to find the critical points of the function, which are the points where the derivative is zero or undefined. We then use the first or second derivative test to determine whether each critical point is a maximum or minimum.

For example, suppose we want to find the maximum area of a rectangle with a fixed perimeter of 20 units. Let the length of one side of the rectangle be x, and the length of the other side be y. Then we have:

```
2x + 2y = 20 (border is settled)
xy = A (zone)
```

Solving for y in terms of x and substituting into the area formula,, we get:

```
y = 10 - x
A = xy = x(10 - x) = 10x - x^2
```

The derivative of A with regard to x is:

```
dA/dx = 10 - 2x
```

Setting this equal to zero and solving for x, we get x = 5. This is a critical point of A. To determine whether it is a maximum or minimum, we use the second derivative test:

```
d^2A/dx^2 = -2
```

Since this is negative at x = 5, we conclude that A has a local maximum at x = 5. Therefore, the maximum area of the rectangle is A = 25 square units, when x = y = 5.

2. **Related rates issues: Finding rates of alter of related variables**

Another important application of derivatives is in related rates problems, where we want to find the rate of change of one variable with respect to time, given the rate of change of one or more related variables. For example, if a spherical balloon is being inflated at a certain rate, we might want to find the rate at which the volume of the balloon is increasing.

To solve these problems, we use the chain rule to differentiate both sides of an equation that relates the variables, and then solve for the desired rate of change.

For example, suppose a spherical balloon is being inflated at a rate of 4 cubic inches per second, and we want to find the rate at which the radius of the balloon is increasing when the radius is 2 inches. The volume V of a sphere with radius r is given by:

```
V = (4/3)πr^3
```

Differentiating both sides with respect to time using the chain rule, we get:

```
dV/dt = (4/3)π(3r^2)(dr/dt)
```

At the given instant when r = 2, we have:

```
dV/dt = 4
r = 2
dr/dt = ?
```

Substituting these values into the equation above, we get:

```
4 = (4/3)π(3(2^2))(dr/dt)
```

Simplifying and solving for dr/dt, we get:

dr/dt = 1/3π cubic inches per moment

Therefore, the radius of the balloon is increasing at a rate of 1/3π cubic inches per second when the radius is 2 inches.

3. **Derivatives and the shape of graphs**

Derivatives can also be used to analyze the shape of graphs and make predictions about their behaviour. For example, we can use derivatives to find critical points, which are points where the derivative is zero or undefined. At a critical point, the function may have a maximum or minimum value, or it may be a point of inflection, where the concavity of the function changes.

To analyze the concavity of a function, we look at the second derivative. If the second derivative is positive, the function is concave up, meaning it is shaped like a cup. If the second derivative is negative, the function is concave down, meaning it is shaped like a bowl. If the second derivative is zero, we need to look at higher-order derivatives to determine the concavity.

For example, consider the function f(x) = x^3 - 3x^2 + 2x. The first derivative is:

```
f(x) = x^3 - 3x^2 + 2x
```

The first derivative is:

```
f'(x) = 3x^2 - 6x + 2
```

Setting this equal to zero and solving for x, we get two critical points: x = 1 and x = 2/3. To determine whether each critical point is a maximum or minimum, we use the second derivative test:

```
f''(x) = 6x - 6
```

At x = 1, f''(x) = 0, so we have to be look at the third derivative:

`f'''(x) = 6`

Since f'''(1) is positive, we conclude that x = 1 is a point of inflection. At

```
x = 2/3, f''(x) = 6/3 - 6 = -2
```

so we conclude that x = 2/3 is a local maximum.

4. **Applications of derivatives in physics: velocity, acceleration, and related concepts**

Derivatives are also widely used in physics to describe the motion of objects. The velocity of an object is the rate of change of its position with respect to time, while the acceleration is the rate of change of its velocity with respect to time.

For example, suppose a car is moving along a straight road with position function

```
s(t) = t^3 - 6t^2 + 9t
```

where s is measured in meters and t is measured in seconds. The velocity v(t) of the car is given by the derivative of s with respect to time:

```
v(t) = ds/dt = 3t^2 - 12t + 9
```

The acceleration a(t) of the car is given by the derivative of v with respect to time:

```
a(t) = dv/dt = 6t - 12
```

We can use these formulas to answer questions about the motion of the car, such as how fast it is moving at a certain time, or how much it is accelerating.

For example, suppose we want to find the velocity of the car at time t = 2 seconds. Then we have:

```
v(2) = 3(2)^2 - 12(2) + 9 = -3 meters per second
```

This means the car is moving backwards at a speed of 3 meters per second at time t = 2 seconds.

**Conclusion**

In conclusion, derivatives and their applications are a crucial concept in calculus and have numerous practical applications in various fields such as business, physics, and engineering. By using derivatives, we can find the rate of change of a function, analyze its curvature, and solve optimization and related rates problems. Derivatives are also widely used in physics to describe the motion of objects. By understanding the concepts and formulas associated with derivatives, we can solve many problems and make predictions about the behavior of functions and physical systems.

**Key takeaways**

- Derivatives are used to find the rate of change of a function with respect to its variable.
- The derivative of a function can be used to find the slope of its graph at any given point.
- Higher-order derivatives can be used to analyze the curvature of a function's graph.
- Applications of derivatives include optimization, related rates problems, and finding critical points, inflection points, and concavity.
- In physics, derivatives are used to describe the motion of objects, including velocity and acceleration.
- By understanding the concepts and formulas associated with derivatives, we can solve many problems and make predictions about the behavior of functions and physical systems.

**Quiz**

**1. What is the derivative of f(x) = sin(x) with respect to x?**

a) cos(x)

b) -sin(x)

c) tan(x)

d) cot(x)

**Answer**: a) cos(x)

**2. Which of the following is a critical point of the function f(x) = 2x^3 - 9x^2 + 12x + 5?**

a) x = 1

b) x = 2

c) x = 3

d) x = 4

**Answer**: b) x = 2

**3. A rectangle with perimeter 20 units has the maximum possible area. What is the length of the rectangle?**

a) 2.5 units

b) 3.33 units

c) 5 units

d) 6.67 units

**Answer**: a) 2.5 units

**4. An object moves along a straight line with velocity v(t) = 5 - 2t, where t is in seconds. What is the acceleration of the object at t = 2 seconds?**

a) -2 m/s^2

b) -5 m/s^2

c) 2 m/s^2

d) 5 m/s^2

**Answer**: b) -5 m/s^2

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