## Introduction to Integral Calculus

The ultimate goal of this section will be to derive the following:

**(a) Formulae for volume** of the cylinder, cone, sphere, and various parabaloids using two methods; the slab (disk) method and the shell method.

**(b) Formulae for surface area** of the cone and sphere using similar methods to those mentioned above.

**(c) Formulae for area** of the circle and ellipse.

**(d) Formula for arc length** of a circle given its central angle. For this, we will reference the Mean Value Theorem and our knowledge of differentiaion.

Along the way we will be introduced to several methods of integration, all of which will be called upon when needed in meeting each of our goals stated above.

I will be developing notes once again over the course of the next several weeks addressing this topic. These notes will be hand-written, scanned and saved as photo files. I will place links at appropriate intervals throughout this page that will lead you to those notes. I will also include other links that will serve to reinforce the contents of my notes; these links will be added to the sidebar of my Math 31 blog for easy referencing if you choose to search them out at a later date.

**POWER RULE OF INTEGRATION**

Integration and differentiation are inverse processes of one another. Differential Calculus was not the first of the two to be developed but is typically easier to learn; that is why it is taught prior to its Integral counterpart. Differential Calculus is often referenced when integrating functions because of this. Having said that, I have attempted to explain Integral Calculus on its own with minimal reference to differentiation. I did “slip” and found myself making just such a reference on page 12 in an attempt to make a justification. Hopefully I will be fogiven for this small indiscretion. The link directly below contains 12 pages of hand-written notes illustrating how I introduce the concept of integration. More notes will follow over the next several weeks as we progress through more complex functions.

Introduction to Integral Calculus – Samuelson

There are many very good references availble to further assist in the understanding of this topic. The information contained in the links below will resemble much of what is contained in my notes and ellaborate further on those points.

The Fundamental Theorem of Calculus

Anti-Differentiation & the Power Rule

Antiderivatives/Indefinite Integrals

Before we can derive the formula for volume (or area) of anything, a thorough understanding of the process of integration is required. There are several different methods of integration available to us; the method chosen depends on the type of function being integrated. We will explore these various types of functions and the appropriate method of integration to be used in each case; upon completion of this requirement, we will be equipped to reach our goal.

**INTEGRATION BY PARTS**

Let us reflect back on the process of differentiation for a few moments. This entire process began with our wish to find an expression representing instantaneous velocity at any given point on a function; we found that this value was represented by the slope of the tangent to the function at that point. The expression for this was discovered through first principles (delta method), and is now referred to as the derivative of the function; we used first principles to find derivatives of several different types of functions in order to more fully conceptualize the notion. As our functions became more complex, this process became much more arduous and time consuming so we were introduced to some helpful **rules **to assist us in our work; they are the power rule, the product/quotient rule, and the chain rule.

Integration is essentially the “inverse” process of differentiaion and has its own set of “rules” that can assist us in determining the area, among other things, under a curve in a given interval. We have already been introduced to the power rule for integration earlier on this page. The power rule works very well for relatively simple functions; as these functions become more complex the neccessity for other rules emerges, just as it did with differential calculus. The next “rule” that we will familiarize ourselves with is referred to as **Integration by Parts**,** **a process tied directly to the product rule of differentiation; the notes contained in the link below will illustrate this. In these notes, a comparison between integration using the **power rule** and **integration by parts** will be made early on as they relate to simple polynomial funtions. The purpose behind this is to help us become accustomed to this new “rule”; as we work through the notes, the functions being integrated will evolve into more complex ones, hopefully leaving us with a deeper appreciation for this new process.

Integration by Parts – Samuelson

For additional information on **Integration by Parts**, click on the links below. For easy reference, these links can also be found on my Math 31 Blog in the sidebar under Integral Calculus.

**INTEGRATION BY PARTIAL FRACTIONS**

We are now ready to move on to the next stage of our journey, that being **Integration by Partial Fractions.** This process often results in a situation requiring the natural logarithm and hence, some knowledge of its inverse, the Euler constant. That being the case, a brief overview of those concepts would be appropriate at this time. In the links directly below, James Tanton first illustrates Euler’s constant (e), relating it to the compound interest formula. This is followed by the derivation of Euler’s Formula by means other than the Taylor Series; although this is not a requirement at the high school level, it is nonetheless very enlightening and should be viewed by all. Beneath Tanton’s offerings are two additional links, one providing an overview of the natural logarithm; the final link directly below provides a description of Integration by Partial Fractions. A small investment of time spent in each of these will serve us well as we proceed on through this process.

Integration by Partial Fractions

The link below contains 23 pages of hand-written notes describing the Integration of Rational Functions. The focus of these notes is centered on the decomposition of rational functions into the sum of partial fractions; all but one of the examples chosen require the introduction of the natural logarithm to the integration process. For this reason, the first several pages in this set of notes focus on that function and its inverse, y = e^x. The example shown on pages 20 and 21 does not require the natural logarithm; it is integrated by Trigonometric substitution instead, a process that will be explained at a later date. For now we will work towards the mastery of Integration by Partial Fractions.

Integration by Partial Fractions – Samuelson

**INTEGRATION BY TRIGONOMETRIC SUBSTITUTION**

We are now within reach of fulfilling our goals stated at the outset; these goals are to derive the following:

**(a) Formulae for volume** of the cylinder, cone, sphere, and various parabaloids using two methods; the slab (disk) method and the shell method.

**(b) Formulae for surface area** of the cone and sphere using similar methods to those mentioned above.

**(c) Formulae for area** of the circle and ellipse.

**(d) Formula for arc length** of a circle given its central angle. For this, we will reference the Mean Value Theorem and our knowledge of differentiaion.

Before proceeding any further a “new” method of integration must be introduced, that being Integration by Trigonometric Substitution; this was eluded to in the previous set of notes. The example illustrated on pages 20 & 21 in my notes on Integration by Partial Fractions revealed that the antiderivative of the expression “[3/(x^2+1)]” was “[3arctan(x)]“. This method will be explained in my next set of notes; we will also be made aware of why the “Natural Logarithm Method” **WILL NOT** work in finding the antiderivative of this expression. My notes on Integration by Trigonometric Substitution can be found in the link directly below.

Integration by Trigonometric Substitution – Samuelson

Visit the links below for additional information on this method of integration. The first one takes you to a lecture given by a guest instructor at MIT.

Trigonometric Substitution & Polar Coordinates

Integration by Trigonometric Substitution (1)

Integration by Trigonometric Substitution (2)

Integrating Algebraic Functions

**CIRCLE AREA & ARC LENGTH**

The notes contained in the link directly below show the derivation of circle area using some of the integration techniques illustrated earlier; the formula for arc length is also derived, with reference to the Mean Value Theorem. This formula is applied to several functions to determine arc length over a given interval and is ultimately used to prove the formula for circumference of a circle. This circumference formula is then used to once again prove the formula for area of circles, this time using the “shell” method of integration.

Circle Area & Arc Length – Samuelson

The links below reinforce the ideas presented in my notes and provide much more information on those topics.

Mean Value Theorem – Interactive

Arc Length, Area, and the Arcsine Function

**SURFACE AREAS & VOLUMES OF REVOLUTION**

When the graph of a 2-dimensional function is revolved about a vertical or horizontal axis, the result is referred to as a 3-dimensional solid. By applying some of the concepts we have already learned, the surface areas and volumes of many such solids of revolution can be determined with relative ease. The notes contained in the link directly below call on our knowledge of arc length to assist in determining the area of surfaces of revolution; direct reference is made here to the “onion proof” that was introduced in the previous set of notes. The premise behind this approach is to find the sum of the areas of infinitely many, infinitely narrow “bands” that are “wrapped” around the surface of revolution. If one of these “bands” was cut and then stretched out, it would form a rectangle whose length would be equivalent to the circumference (arc length) of the surface of revolution at that point; the width of this “band” would ultimately be represented by “dx” or “dy”, depending on the axis of revolution.

Several examples involving the calculation of surface areas are illustrated in my notes; following those are two methods of determining volumes of solids of revolution, the first being the “Shell Method”. This method once again reflects back on the “onion proof” that was used previouly to derive the formula for area of a circle. The idea behind the “onion proof” is that “layers” (bands) are added uniformly around the circumference until the desired radius is attained. The areas of these infinitely narrow “layers” are then added to determine the area of the circle; this circle essentially forms the base of the cylindrical shells that emerge as a result of revolving a function about an axis. The “height” of these cylindrical shells is determined by the function itself and is always perpendicular to its circular base; the volume of each cylindrical shell is the product of its “height” and the area of its circular base. As layers are added to the circular base, the “height” of these cylindrical shells also changes, governed by the function itself; the volumes of these cylindrical shells, having infinitely thin lateral surfaces, are ultimately added to determine the volume of the solid itself. After working through several examples using the Shell Method, the volumes of many of the same solids are calculated using the Slab (Disk) Method. With this method, cylindrical “slabs” of infinitely small “height” are stacked together, their individual volumes added to determine the volume of the solid of revolution.

Surface Areas & Volumes of Revolution – Samuelson

Additional links have been included below that will reinforce (and go beyond) the notions presented above.

Area of a Surface of Revolution