Summary of Calculus
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Summary of Calculus William G. Faris December 6, 2004 2 Chapter 1 Functions 1.1 A catalog of functions A function f takes an input number in its domain and gives an output number in its range. If for each output number in the range there is only one corresponding number in the domain, then the function f has an inverse function f −1. That is, if y = f(x), then x is defined in terms of y by x = f −1(y). The domain of the inverse function is the range of the original function, and the range of the inverse function is the domain of the original function. Sometimes a function may not have an inverse function, but by restricting it to a smaller domain it will have an inverse function. In that case, the inverse function is determined by solving y = f(x) for x, with x in this restricted domain. For example, the squaring function y = x2 is naturally defined on the domain of all real numbers x, and it does not have an inverse function. However if we restrict the squaring function to the smaller domain of all x ≥ 0, then there 1 is an inverse function x = y 2 , that is, taking the positive square root. In mathematics it is customary to describe a function by what it does on input values. In a few cases there are explicit names for the functions. For in- stance, many calculators and computer languages have notations such as square and sqrt that describe the function itself. Thus for example the square func- tion is the function that sends x to square(x) = x2. Similarly, the square root p 1 function sqrt sends x to sqrt(x) = x = x 2 . A function is often described by a graph, where ordinarily the horizontal axis represents the input, and the vertical axis represents the output. For the graph to describe a function, it must have the property that every vertical line intersects the graph at most once. For the graph to describe a function with an inverse, it must have the property that every horizontal line intersects the graph at most once. The graph of the inverse function is obtained by interchanging the roles of horizontal and vertical. For a first example of a function, fix a number p called the power. If x is the input number, then xp is the output number. This power function may be 3 4 CHAPTER 1. FUNCTIONS defined on the domain consisting of all numbers x > 0, that is, on the interval (0; +1). If p > 0 it may be defined on a larger domain consisting of all real numbers x ≥ 0, that is, on the interval [0; +1). In either case it sends the domain to itself. With these domains the function has an inverse function. 1 1 Since y = xp, implies x = y p , this inverse function sends y to y p . Thus the inverse function of the pth power function is the 1=pth power function. In some circumstances the power function may be defined on a larger domain. Say that p = n=k, where n and k are integers, and k is odd. Then the domain may be extended to include all numbers x < 0, that is, the interval (−∞; 0) is a subset of the domain. If n is odd, then the function has the range equal to 1 the domain. If n is odd, the inverse function sends y to y p , where 1=p = k=n. If n is even, then the range is a subset of [0; 1). In this case the function with this larger domain has no inverse function. For a second example, fix a number a > 0 called the base. To make the function interesting, take a 6= 1. If x is the input number, then ax is the output number. This is the exponential function with base a. The domain consists of all real numbers, and the range consists of the interval (0; +1). Since y = ax is equivalent to x = loga(y), the inverse of the exponential function with base a is the logarithm function with base a. The most common choices of base are 2, e, and 10. The almost universal choice in calculus contexts is a = e. In this case, the exponential function is sometimes denoted exp, and the logarithm function is often denoted ln. Thus exp−1 = ln : (1.1) The reason for this choice is that e is the only base for which the exponential function satisfies the inequality 1 + x ≤ ex (1.2) for all x. All the other exponential functions may be defined in terms of the one with base e. This is because ax = (eln(a))x = eln(a)x. All one has to know is the numerical value of the constant ln(a). Similarly, if y = loga(x), then x = y ln(a)y a = e , so ln(a)y = ln(x), that is, loga(x) = ln(x)= ln(a). Again the same constant is involved. Notice that the inequality for other bases a > 1 takes the form 1 + ln(a)t ≤ at (1.3) for all t. The reason for the simplicity in the case of base e is that ln(e) = 1. The next example is that of the trigonometric functions. These functions will always be defined with radians as inputs. (Degrees may be converted to radians by multiplying by π=180.) The functions sin and cos have domain consisting of all real numbers and range consisting of the interval [−1; 1]. With these natural domains they do not have inverse functions. However if one restricts sin to [−π=2; π=2] and cos to [0; π], then the restricted functions have inverses, called arcsin and arccos. Thus sin−1 = arcsin (1.4) 1.2. EXPONENTIALS BEAT POWERS 5 and cos−1 = arccos : (1.5) The tangent function tan is defined by dividing the output of the sin function by the output of the cosine function. Since the cosine is zero at odd multiples of π=2, the domain of the tangent function must exclude these points. The tangent function does not have an inverse unless it is restricted to a smaller interval, and the natural restriction is to (−π=2; π=2). Then inverse function is tan−1 = arctan : (1.6) A final example of a function is a constant function. This gives the constant output c for every input. 1.2 Exponentials beat powers An exponential function will always be larger than a power function for suffi- ciently large input values. In fact, from 1 + x ≤ ex we see that for x ≥ 0 we have (1 + x)n ≤ enx for each n = 1; 2; 3;:::. We may set t = nx and get t (1 + )n ≤ et (1.7) n for t ≥ 0. This says that the exponential function grows at least as fast as an 1 2 t nth degree polynomial. For example, when n = 2 we have 1 + t + 4 t ≤ e for t ≥ 0. From 1 + x ≤ ex we get 1 + ln(y) ≤ y for all y > 0. Set y = sp for p > 0. This gives 1 + p ln(s) ≤ sp, or 1 ln(s) ≤ (sp − 1) (1.8) p for s > 0. This says that the logarithm function grows no faster than a p power 1 p function. For example, when p = 1=2 we have ln(s) ≤ 2 ( s − 1) for s > 0. 1.3 Combining functions Functions may be combined by addition, subtraction, multiplication, and divi- sion. This is done by performing the corresponding operation on the output values. Thus the value of the sum or difference function f ± g at x is (f ± g)(x) = f(x) ± g(x): (1.9) The value of the product function f · g at x is (f · g)(x) = f(x) · g(x): (1.10) 6 CHAPTER 1. FUNCTIONS f The value of the quotient function g at x is f f(x) ( )(x) = : (1.11) g g(x) Here the domain is restricted to those x for which the denominator g(x) 6= 0. Start with the constant functions and the first power function. If we repeat- edly apply the sum, difference, and product operations, we obtain the polynomial functions. If in addition we apply the quotient operations we obtain the rational functions. Another method of combining functions is composition. The composition f ◦ g is defined by (f ◦ g)(x) = f(g(x)): (1.12) Thus the output of g becomes the input of f. In other words, the output is obtained by first applying g and then applying f. The order in which functions are composed is important. Thus, for example, the function sin ◦ square with input x has output sin(x2). On the other hand, the function square ◦ sin with input x has output (sin(x))2. In general, the composition f ◦ g represents the action of g followed by the action of f. The notion of inverse function is closely related to the notion of composition. If f −1 is the inverse function to f, so that y = f(x) is equivalent to x = f −1(y), then f −1(f(x)) = x and f(f −1(y)) = y. Thus arcsin(sin(x)) = x and sin(arcsin(y)) = y, where −π=2 ≤ x ≤ π=2 and −1 ≤ y ≤ 1. It is extremely important to avoid confusion between the notation g−1(x) for inverse function and the notation g(x)−1 = 1=g(x) for reciprocal function. These both play an important role in calculus, and the fact that the notation is essentially the same requires constant vigilance.