Lecture 24 Notes: Inverse Trigonometric Functions and Hyperbolic Functions

18.01 Calculus Jason Starr Fall 2005 Lecture 24. November 15, 2005 Practice Problems. Course Reader: 5A­1, 5A­2, 5A­3, 5A­5, 5A­6. 1. Inverse functions. Let a, b, s and t be constants. Let y = f (x) be a function defined on the interval, a ≤ x ≤ b, and whose values are in the interval, s ≤ y ≤ t. Does there exist a function x = g(y) defined on the interval, s ≤ y ≤ t, whose values are in the interval, a ≤ x ≤ b, satisfying the two conditions, g(f (x)) = x, f (g(y)) = y ? If such a function g exists, it is called an inverse function of f , and it is denoted by f −1 (y). Also, the original function f (x) is called invertible. There is some chance of confusion with the other use 18.01 Calculus Jason Starr Fall 2005 of “invertible”, namely that 1/f (x) is always defined. We will be careful to specify the meaning of “invertible”. There are 2 necessary conditions for f to have an inverse function. Assume f has an inverse function g. Let x1 , x2 be a pair of numbers in [a, b]. If f (x1 ) equals f (x2 ), then also, x1 = g(f (x1 )) = g(f (x2 )) = x2 , i.e., x1 equals x2 . In other words, two distinct inputs x1 and x2 give two distinct outputs f (x1 ) and f (x2 ). A function satisfying this condition is called one­to­one, because to every output, there is at most one input. This is the first necessary condition: every invertible function is one­to­one. Next, for every number y in [s, t], there is a number x in [a, b] such that y = f (x). In fact, just take x to be g(y); then f (x) equals f (g(y)), which equals y. A function satisfying this condition is called onto. This is the second necessary condition: every invertible function is onto. Together, this says that an invertible function is one­to­one and onto. In fact, the converse is also true: every one­to­one and onto function is invertible. This is easy to check, but we will not prove it in this class. Remark: In checking that f is one­to­one and onto, the choice of intervals [a, b] and [c, d] are vital. A simple example comes from f (x) = sin(x). For the interval [−π/2, π, 2] and [−1, 1], the function f (x) is one­to­one and onto. But for many other choices of these intervals, the function is neither one­to­one nor onto. 2. The graph of an inverse function. How should we think of an inverse function? One way is graphically. The graph of the function y = f −1 (x) is the same as the graph of f (y) = x. This is simply the usual graph of y = f (x) with the roles of x and y reversed. What this translates to is, the graph of f −1 is the same as the graph of f with the roles of the x­axis and y­axis reversed. The simplest way to get the graph of f −1 (x) is simply to reflect the graph of f (x) through the 45◦ line y = x. 3. The inverse trigonometric functions. The function sin(x) is one­to­one and onto on [−π/2, π/2], taking values in [−1, 1]. Thus there is an inverse function sin−1 (x) defined on the interval [−1, 1], taking values in [−π/2, π/2]. The graph of sin−1 (x) is an increasing function whose lower left endpoint is (−1, −π/2) and whose upper right endpoint is (1, π/2). The function cos(x) is one­to­one and onto on [0, π], taking values in [−1, 1]. Thus there is an inverse function cos−1 (x) defined on the interval [−1, 1], taking values in [0, π]. The graph of cos−1 (x) is a decreasing function whose upper left endpoint is (−1, π) and whose lower right endpoing is (1, 0). The function tan(x) is one­to­one and onto on (−π/2, π/2), taking values in the whole real line. Thus there is an inverse function tan−1 (x) defined on the whole real line, taking values in (−π/2, π/2). The graph is an increasing function that is asymptotic to the line y = −π/2 as x → −∞, and asymptotic to the line y = +π/2 as x → +∞. 4. Derivatives of inverse functions. A particular simple formulation of the chain rule is the differential formulation, df (u) = f � (u)du. 18.01 Calculus Jason Starr Fall 2005 If f has an inverse function g(x), let u be g(x). Then this gives, df (g(x)) = f � (g(x))dg(x). On the other hand, f (g(x)) equals x. This gives the formula, dx = f � (g(x))dg(x). Solving for dg/dx gives, d (g(x)) = 1/f � (g(x)). dx This is the formula for the derivative of an inverse function. In fact, we have seen this formula before. It is how we computed the derivative of ln(x), the inverse function of ex : d 1 1 (ln(x)) = ln(x) = . dx e x 5. Derivatives of the inverse trigonometric functions. Because the derivative of sin(x) is cos(x), the formula above gives, d 1 (sin−1 (x)) = . dx cos(sin−1 (x)) This isn’t very useful. A simple argument makes it much more useful. Denote sin−1 (x) by θ. Thus sin(θ) = x. Also, the formula for the derivative is a bit simpler, 1 d (sin−1 (x)) = . dx cos(θ) By the Pythagorean theorem, sin2 (θ) + cos2 (θ) = 1. Solving gives, � √ cos(θ) = 1 − sin2 (θ) = 1 − x2 . This gives a very useful formula for the derivative of sin−1 (x), √ d (sin−1 (x)) = 1/ 1 − x2 . dx There is a very similar derivation that, √ d (cos−1 (x)) = −1/ 1 − x2 . dx 18.01 Calculus Jason Starr Fall 2005 This looks remarkably similar to the previous formula. In particular, this gives, 1 d −1 (sin−1 (x) + cos−1 (x)) = √ +√ = 0. dx 1 − x2 1 − x2 Therefore the sum is a constant function. Checking at x = 0 gives the value of this constant function, sin−1 (x) + cos−1 (x) = π/2. Finally, because the derivative of tan(x) is sec2 (x), the formula gives, d 1 (tan−1 (x)) = . 2 dx sec (tan−1 (x)) Again introduce θ = tan−1 (x). Then the formula for the derivative is, d 1 . (tan−1 (x)) = dx sec2 (θ) But the Pythagorean theorem implies, sec2 (θ) = 1 + tan2 (θ) = 1 + x2 . This finally gives a very useful formula for the derivative of tan(x), d (tan−1 (x)) = 1/(1 + x2 ). dx Notice, in particular, that the denominator is never zero. This is closely related to the fact that tan−1 (x) is defined on the entire real line. 6. Hyperbolic trigonometric functions. The trigonometric functions are very useful for dis­ cussing point on the unit circle x2 + y 2 = 1, because the circle is the parametric curve, � x = cos(θ), y = sin(θ) Are there analogous continuous functions for the points on the hyperbola x2 − y 2 = 1? At first blush, the answer is no. The problem is that the hyperbola has two parts: one part is in the half­plane where x > 0, and the other part is in the half­plane where x < 0. Because of the intermediate value theorem, a continuous function x = f (t) cannot jump from x > 0 to x < 0 or vice versa without crossing x = 0. Thus, refine the question: Are there continuous functions for the part of the hyperbola in the half­plane where x > 0? The answer to this question is yes. The corresponding functions are called hyperbolic trigonometric functions or, more often, simply hyperbolic functions. They are defined as follows, 1 cosh(t) = (et + e−t ), 2 18.01 Calculus Jason Starr Fall 2005 1 sinh(t) = (et − e−t ), 2 sinh(t) et − e−t tanh(t) = = t , cosh(t) e + e−t 1 2 sech(t) = = t , cosh(t) e + e−t 1 2 csch(t) = = t , sinh(t) e − e−t and, coth(t) = 1 cosh(t) et + e−t = = t . tanh(t) sinh(t) e − e−t The first observation is that, 1 1 cosh2 (t) = (et + e−t )2 = (e2t + 2 + e−2t ), 4 4 1 1 sinh2 (t) = (et − e−t )2 = (e2t − 2 + e−2t ). 4 4 Taking the difference of these, most of the terms cancel, 1 4 cosh2 (t) − sinh2 (t) = ((2) − (−2)) = = 1. 4 4 This proves that the parametric curve, � x = cosh(t), y = sinh(t) is contained in the right­half of the hyperbola x2 − y 2 = 1. We will see next time that there is an inverse function of sinh(t), from which it follows that every point in the right­half of the hyperbola occurs for exactly one value of t. Thus the parametric curve exactly traces out the right­half of the hyperbola. 7. The derivatives of the hyperbolic functions. The derivatives of the hyperbolic functions are straightforward. The formulas are very similar to the formulas in the trigonometric case, but slightly different. Try not to confuse them. d (sinh(x)) = cosh(x). dx d (cosh(x)) = sinh(x). dx d 1 1 (tanh(x)) = (cosh(x) · cosh(x) − sinh(x) · sinh(x)) = = sech2 (x). 2 2 dx cosh (x) cosh (x)