Lecture 10: Linear Approximation
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Linear Approximation of the Derivative
Section 3.9 - Linear Approximation and the Derivative Idea: When we zoom in on a smooth function and its tangent line, they are very close to- gether. So for a function that is hard to evaluate, we can use the tangent line to approximate values of the derivative. p 3 Example Usep a tangent line to approximatep 8:1 without a calculator. Let f(x) = 3 x = x1=3. We know f(8) = 3 8 = 2. We'll use the tangent line at x = 8. 1 1 1 1 f 0(x) = x−2=3 so f 0(8) = (8)−2=3 = · 3 3 3 4 Our tangent line passes through (8; 2) and has slope 1=12: 1 y = (x − 8) + 2: 12 To approximate f(8:1) we'll find the value of the tangent line at x = 8:1. 1 y = (8:1 − 8) + 2 12 1 = (:1) + 2 12 1 = + 2 120 241 = 120 p 3 241 So 8:1 ≈ 120 : p How accurate was this approximation? We'll use a calculator now: 3 8:1 ≈ 2:00829885. 241 −5 Taking the difference, 2:00829885 − 120 ≈ −3:4483 × 10 , a pretty good approximation! General Formulas for Linear Approximation The tangent line to f(x) at x = a passes through the point (a; f(a)) and has slope f 0(a) so its equation is y = f 0(a)(x − a) + f(a) The tangent line is the best linear approximation to f(x) near x = a so f(x) ≈ f(a) + f 0(a)(x − a) this is called the local linear approximation of f(x) near x = a. -
Generalized Finite-Difference Schemes
Generalized Finite-Difference Schemes By Blair Swartz* and Burton Wendroff** Abstract. Finite-difference schemes for initial boundary-value problems for partial differential equations lead to systems of equations which must be solved at each time step. Other methods also lead to systems of equations. We call a method a generalized finite-difference scheme if the matrix of coefficients of the system is sparse. Galerkin's method, using a local basis, provides unconditionally stable, implicit generalized finite-difference schemes for a large class of linear and nonlinear problems. The equations can be generated by computer program. The schemes will, in general, be not more efficient than standard finite-difference schemes when such standard stable schemes exist. We exhibit a generalized finite-difference scheme for Burgers' equation and solve it with a step function for initial data. | 1. Well-Posed Problems and Semibounded Operators. We consider a system of partial differential equations of the following form: (1.1) du/dt = Au + f, where u = (iti(a;, t), ■ ■ -, umix, t)),f = ifxix, t), ■ • -,fmix, t)), and A is a matrix of partial differential operators in x — ixx, • • -, xn), A = Aix,t,D) = JjüiD', D<= id/dXx)h--Yd/dxn)in, aiix, t) = a,-,...i„(a;, t) = matrix . Equation (1.1) is assumed to hold in some n-dimensional region Í2 with boundary An initial condition is imposed in ti, (1.2) uix, 0) = uoix) , xEV. The boundary conditions are that there is a collection (B of operators Bix, D) such that (1.3) Bix,D)u = 0, xEdti,B<E(2>. We assume the operator A satisfies the following condition : There exists a scalar product {,) such that for all sufficiently smooth functions <¡>ix)which satisfy (1.3), (1.4) 2 Re {A4,,<t>) ^ C(<b,<p), 0 < t ^ T , where C is a constant independent of <p.An operator A satisfying (1.4) is called semi- bounded. -
An Introduction to Mathematical Modelling
An Introduction to Mathematical Modelling Glenn Marion, Bioinformatics and Statistics Scotland Given 2008 by Daniel Lawson and Glenn Marion 2008 Contents 1 Introduction 1 1.1 Whatismathematicalmodelling?. .......... 1 1.2 Whatobjectivescanmodellingachieve? . ............ 1 1.3 Classificationsofmodels . ......... 1 1.4 Stagesofmodelling............................... ....... 2 2 Building models 4 2.1 Gettingstarted .................................. ...... 4 2.2 Systemsanalysis ................................. ...... 4 2.2.1 Makingassumptions ............................. .... 4 2.2.2 Flowdiagrams .................................. 6 2.3 Choosingmathematicalequations. ........... 7 2.3.1 Equationsfromtheliterature . ........ 7 2.3.2 Analogiesfromphysics. ...... 8 2.3.3 Dataexploration ............................... .... 8 2.4 Solvingequations................................ ....... 9 2.4.1 Analytically.................................. .... 9 2.4.2 Numerically................................... 10 3 Studying models 12 3.1 Dimensionlessform............................... ....... 12 3.2 Asymptoticbehaviour ............................. ....... 12 3.3 Sensitivityanalysis . ......... 14 3.4 Modellingmodeloutput . ....... 16 4 Testing models 18 4.1 Testingtheassumptions . ........ 18 4.2 Modelstructure.................................. ...... 18 i 4.3 Predictionofpreviouslyunuseddata . ............ 18 4.3.1 Reasonsforpredictionerrors . ........ 20 4.4 Estimatingmodelparameters . ......... 20 4.5 Comparingtwomodelsforthesamesystem . ......... -
Example 10.8 Testing the Steady-State Approximation. ⊕ a B C C a B C P + → → + → D Dt K K K K K K [ ]
Example 10.8 Testing the steady-state approximation. Å The steady-state approximation contains an apparent contradiction: we set the time derivative of the concentration of some species (a reaction intermediate) equal to zero — implying that it is a constant — and then derive a formula showing how it changes with time. Actually, there is no contradiction since all that is required it that the rate of change of the "steady" species be small compared to the rate of reaction (as measured by the rate of disappearance of the reactant or appearance of the product). But exactly when (in a practical sense) is this approximation appropriate? It is often applied as a matter of convenience and justified ex post facto — that is, if the resulting rate law fits the data then the approximation is considered justified. But as this example demonstrates, such reasoning is dangerous and possible erroneous. We examine the mechanism A + B ¾¾1® C C ¾¾2® A + B (10.12) 3 C® P (Note that the second reaction is the reverse of the first, so we have a reversible second-order reaction followed by an irreversible first-order reaction.) The rate constants are k1 for the forward reaction of the first step, k2 for the reverse of the first step, and k3 for the second step. This mechanism is readily solved for with the steady-state approximation to give d[A] k1k3 = - ke[A][B] with ke = (10.13) dt k2 + k3 (TEXT Eq. (10.38)). With initial concentrations of A and B equal, hence [A] = [B] for all times, this equation integrates to 1 1 = + ket (10.14) [A] A0 where A0 is the initial concentration (equal to 1 in work to follow). -
Differentiation Rules (Differential Calculus)
Differentiation Rules (Differential Calculus) 1. Notation The derivative of a function f with respect to one independent variable (usually x or t) is a function that will be denoted by D f . Note that f (x) and (D f )(x) are the values of these functions at x. 2. Alternate Notations for (D f )(x) d d f (x) d f 0 (1) For functions f in one variable, x, alternate notations are: Dx f (x), dx f (x), dx , dx (x), f (x), f (x). The “(x)” part might be dropped although technically this changes the meaning: f is the name of a function, dy 0 whereas f (x) is the value of it at x. If y = f (x), then Dxy, dx , y , etc. can be used. If the variable t represents time then Dt f can be written f˙. The differential, “d f ”, and the change in f ,“D f ”, are related to the derivative but have special meanings and are never used to indicate ordinary differentiation. dy 0 Historical note: Newton used y,˙ while Leibniz used dx . About a century later Lagrange introduced y and Arbogast introduced the operator notation D. 3. Domains The domain of D f is always a subset of the domain of f . The conventional domain of f , if f (x) is given by an algebraic expression, is all values of x for which the expression is defined and results in a real number. If f has the conventional domain, then D f usually, but not always, has conventional domain. Exceptions are noted below. -
Calculus I - Lecture 15 Linear Approximation & Differentials
Calculus I - Lecture 15 Linear Approximation & Differentials Lecture Notes: http://www.math.ksu.edu/˜gerald/math220d/ Course Syllabus: http://www.math.ksu.edu/math220/spring-2014/indexs14.html Gerald Hoehn (based on notes by T. Cochran) March 11, 2014 Equation of Tangent Line Recall the equation of the tangent line of a curve y = f (x) at the point x = a. The general equation of the tangent line is y = L (x) := f (a) + f 0(a)(x a). a − That is the point-slope form of a line through the point (a, f (a)) with slope f 0(a). Linear Approximation It follows from the geometric picture as well as the equation f (x) f (a) lim − = f 0(a) x a x a → − f (x) f (a) which means that x−a f 0(a) or − ≈ f (x) f (a) + f 0(a)(x a) = L (x) ≈ − a for x close to a. Thus La(x) is a good approximation of f (x) for x near a. If we write x = a + ∆x and let ∆x be sufficiently small this becomes f (a + ∆x) f (a) f (a)∆x. Writing also − ≈ 0 ∆y = ∆f := f (a + ∆x) f (a) this becomes − ∆y = ∆f f 0(a)∆x ≈ In words: for small ∆x the change ∆y in y if one goes from x to x + ∆x is approximately equal to f 0(a)∆x. Visualization of Linear Approximation Example: a) Find the linear approximation of f (x) = √x at x = 16. b) Use it to approximate √15.9. Solution: a) We have to compute the equation of the tangent line at x = 16. -
PLEASANTON UNIFIED SCHOOL DISTRICT Math 8/Algebra I Course Outline Form Course Title
PLEASANTON UNIFIED SCHOOL DISTRICT Math 8/Algebra I Course Outline Form Course Title: Math 8/Algebra I Course Number/CBED Number: Grade Level: 8 Length of Course: 1 year Credit: 10 Meets Graduation Requirements: n/a Required for Graduation: Prerequisite: Math 6/7 or Math 7 Course Description: Main concepts from the CCSS 8th grade content standards include: knowing that there are numbers that are not rational, and approximate them by rational numbers; working with radicals and integer exponents; understanding the connections between proportional relationships, lines, and linear equations; analyzing and solving linear equations and pairs of simultaneous linear equations; defining, evaluating, and comparing functions; using functions to model relationships between quantities; understanding congruence and similarity using physical models, transparencies, or geometry software; understanding and applying the Pythagorean Theorem; investigating patterns of association in bivariate data. Algebra (CCSS Algebra) main concepts include: reason quantitatively and use units to solve problems; create equations that describe numbers or relationships; understanding solving equations as a process of reasoning and explain reasoning; solve equations and inequalities in one variable; solve systems of equations; represent and solve equations and inequalities graphically; extend the properties of exponents to rational exponents; use properties of rational and irrational numbers; analyze and solve linear equations and pairs of simultaneous linear equations; define, -
Chapter 3, Lecture 1: Newton's Method 1 Approximate
Math 484: Nonlinear Programming1 Mikhail Lavrov Chapter 3, Lecture 1: Newton's Method April 15, 2019 University of Illinois at Urbana-Champaign 1 Approximate methods and asumptions The final topic covered in this class is iterative methods for optimization. These are meant to help us find approximate solutions to problems in cases where finding exact solutions would be too hard. There are two things we've taken for granted before which might actually be too hard to do exactly: 1. Evaluating derivatives of a function f (e.g., rf or Hf) at a given point. 2. Solving an equation or system of equations. Before, we've assumed (1) and (2) are both easy. Now we're going to figure out what to do when (2) and possibly (1) is hard. For example: • For a polynomial function of high degree, derivatives are straightforward to compute, but it's impossible to solve equations exactly (even in one variable). • For a function we have no formula for (such as the value function MP(z) from Chapter 5, for instance) we don't have a good way of computing derivatives, and they might not even exist. 2 The classical Newton's method Eventually we'll get to optimization problems. But we'll begin with Newton's method in its basic form: an algorithm for approximately finding zeroes of a function f : R ! R. This is an iterative algorithm: starting with an initial guess x0, it makes a better guess x1, then uses it to make an even better guess x2, and so on. We hope that eventually these approach a solution. -
The Exponential Constant E
The exponential constant e mc-bus-expconstant-2009-1 Introduction The letter e is used in many mathematical calculations to stand for a particular number known as the exponential constant. This leaflet provides information about this important constant, and the related exponential function. The exponential constant The exponential constant is an important mathematical constant and is given the symbol e. Its value is approximately 2.718. It has been found that this value occurs so frequently when mathematics is used to model physical and economic phenomena that it is convenient to write simply e. It is often necessary to work out powers of this constant, such as e2, e3 and so on. Your scientific calculator will be programmed to do this already. You should check that you can use your calculator to do this. Look for a button marked ex, and check that e2 =7.389, and e3 = 20.086 In both cases we have quoted the answer to three decimal places although your calculator will give a more accurate answer than this. You should also check that you can evaluate negative and fractional powers of e such as e1/2 =1.649 and e−2 =0.135 The exponential function If we write y = ex we can calculate the value of y as we vary x. Values obtained in this way can be placed in a table. For example: x −3 −2 −1 01 2 3 y = ex 0.050 0.135 0.368 1 2.718 7.389 20.086 This is a table of values of the exponential function ex. -
The Linear Algebra Version of the Chain Rule 1
Ralph M Kaufmann The Linear Algebra Version of the Chain Rule 1 Idea The differential of a differentiable function at a point gives a good linear approximation of the function – by definition. This means that locally one can just regard linear functions. The algebra of linear functions is best described in terms of linear algebra, i.e. vectors and matrices. Now, in terms of matrices the concatenation of linear functions is the matrix product. Putting these observations together gives the formulation of the chain rule as the Theorem that the linearization of the concatenations of two functions at a point is given by the concatenation of the respective linearizations. Or in other words that matrix describing the linearization of the concatenation is the product of the two matrices describing the linearizations of the two functions. 1. Linear Maps Let V n be the space of n–dimensional vectors. 1.1. Definition. A linear map F : V n → V m is a rule that associates to each n–dimensional vector ~x = hx1, . xni an m–dimensional vector F (~x) = ~y = hy1, . , yni = hf1(~x),..., (fm(~x))i in such a way that: 1) For c ∈ R : F (c~x) = cF (~x) 2) For any two n–dimensional vectors ~x and ~x0: F (~x + ~x0) = F (~x) + F (~x0) If m = 1 such a map is called a linear function. Note that the component functions f1, . , fm are all linear functions. 1.2. Examples. 1) m=1, n=3: all linear functions are of the form y = ax1 + bx2 + cx3 for some a, b, c ∈ R. -
2.8 Linear Approximation and Differentials
196 the derivative 2.8 Linear Approximation and Differentials Newton’s method used tangent lines to “point toward” a root of a function. In this section we examine and use another geometric charac- teristic of tangent lines: If f is differentiable at a, c is close to a and y = L(x) is the line tangent to f (x) at x = a then L(c) is close to f (c). We can use this idea to approximate the values of some commonly used functions and to predict the “error” or uncertainty in a compu- tation if we know the “error” or uncertainty in our original data. At the end of this section, we will define a related concept called the differential of a function. Linear Approximation Because this section uses tangent lines extensively, it is worthwhile to recall how we find the equation of the line tangent to f (x) where x = a: the tangent line goes through the point (a, f (a)) and has slope f 0(a) so, using the point-slope form y − y0 = m(x − x0) for linear equations, we have y − f (a) = f 0(a) · (x − a) ) y = f (a) + f 0(a) · (x − a). If f is differentiable at x = a then an equation of the line L tangent to f at x = a is: L(x) = f (a) + f 0(a) · (x − a) Example 1. Find a formula for L(x), the linear function tangent to the p ( ) = ( ) ( ) ( ) graph of f x p x atp the point 9, 3 . Evaluate L 9.1 and L 8.88 to approximate 9.1 and 8.88. -
Propagation of Error Or Uncertainty
Propagation of Error or Uncertainty Marcel Oliver December 4, 2015 1 Introduction 1.1 Motivation All measurements are subject to error or uncertainty. Common causes are noise or external disturbances, imperfections in the experimental setup and the measuring devices, coarseness or discreteness of instrument scales, unknown parameters, and model errors due to simpli- fying assumptions in the mathematical description of an experiment. An essential aspect of scientific work, therefore, is quantifying and tracking uncertainties from setup, measure- ments, all the way to derived quantities and resulting conclusions. In the following, we can only address some of the most common and simple methods of error analysis. We shall do this mainly from a calculus perspective, with some comments on statistical aspects later on. 1.2 Absolute and relative error When measuring a quantity with true value xtrue, the measured value x may differ by a small amount ∆x. We speak of ∆x as the absolute error or absolute uncertainty of x. Often, the magnitude of error or uncertainty is most naturally expressed as a fraction of the true or the measured value x. Since the true value xtrue is typically not known, we shall define the relative error or relative uncertainty as ∆x=x. In scientific writing, you will frequently encounter measurements reported in the form x = 3:3 ± 0:05. We read this as x = 3:3 and ∆x = 0:05. 1.3 Interpretation Depending on context, ∆x can have any of the following three interpretations which are different, but in simple settings lead to the same conclusions. 1. An exact specification of a deviation in one particular trial (instance of performing an experiment), so that xtrue = x + ∆x : 1 Of course, typically there is no way to know what this true value actually is, but for the analysis of error propagation, this interpretation is useful.