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2.1 Functions: Definition, Notation a Function Is a Rule (Correspondence)

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Chapter 2: 2.1 Functions: definition, notation A function is a rule (correspondence) that assigns to each element x of one set , say X, one and only one element y of another set, Y. The set X is called the domain of the function and the set of all elements of the set Y that are associated with some element of the set X is called the range of the function. For a function described by above diagram, the domain is the set X= {1,2,3,4} and the range is the set {a,b,c}. The elements of a set X are often called inputs and the elements of the set Y are called outputs and a function can be visualized as a machine, that takes x as an input and returns y as an output. The domain of a function is the set of all inputs x that return an output. The range of a function is the set of all outputs y. We will deal with functions for which both domain and the range are the set (or subset) of real numbers Most often functions in mathematics are defined by a formula or by a graph. Very often we define a function by giving a formula or an equation which specifies how the y is obtained when x is given. Example: The equation y = 3x +2 defines a function since we can easily find the output y when the input, x, is given. To obtain the y that correspond to a given x, multiply x by 3 and add 2 to the result. For example, if x is 1 then y =3(1) +2 = 5. So, every x has a corresponding y value. We say that a function is given explicitly by an equation, if the equation is of the form y = (formula that contains only x variable). We say that a function is given implicitly, if it is given by an equation that is not in the above form. For example, y = 3x + 2 is a function given in an explicit form, and 3x - y + 2 = 0 is the same function given in an implicit form. When we use a formula to define a function we often give a name to a function (f,g,h,etc.) and use a special notation for the output, y. If x is the input then we denote the output by f(x) (read “f of x”). Caution: f(x) is not a multiplication of f and x. It is an entity that can’t be split. f(x) denotes output that corresponds to the input x. For example, instead of writing y = 3x+2, we often write f(x) = 3x+2. With this notation f(1) denotes the output (y) that corresponds to the input 1, that is f(1) = 3∙(1)+2= 5. If we say that f(4) = -5, then this means that when the input, x, is 4, the output, y , is -5. An equation defines a function if it can be solved for y and the solution is unique. Example: 3x+2y = 4 is a function, because we can solve it for y, y = (-3/2)x + 2 and the solution is unique. The equation x2 + y2 = 1 is not a function, because when we solve it for y, we get y 1 x 2 , two solutions for -1< x < 1. For example when x = 0, then there are two values of y (± 1) that correspond to x = 0. Hence, this equations does not describe a function. Example: Determine whether the equation 2x + 4y2 -5 = 0 defines y as a function of x. Solution : Let’s solve the equation for y: 4y 2 5 2x 1 y 2 (5 2x) 4 1 y (5 2x) 4 Since there are two solutions, this equation does not define a function The graph of a function is the set of all points (x, f(x)), where x belongs to the domain of f. A point (a,b) is on the graph of a function f, if and only if b = f(a). If f is a function and a is in its domain, then there is one and only one value b that corresponds to a. Therefore, there is only one point on the graph of f that has a as x-coordinate. This leads us to the Vertical Line Test Theorem: If every vertical line crosses the graph at most at one point, then the graph represents a function. The domain of a function given by a graph is the set of all x -coordinates of the points on the graph. The range is the set of all y-coordinates of the points on the graph. Example: Which of the following is the graph of a function? a) b) The graph fails the Vertical Line test, The graph passes the Vertical Line test so the graph does not represent a function so the graph represents a function Example: Determine whether given graph represents a function. Explain why or why not. Solution: We perform the Vertical Line Test, and notice that every vertical line crosses the graph at most at one point. Therefore the graph represents a function. When we use a formula to define a function we often give a name to a function (f,g,h,etc.) and use a special notation for the output, y. If f is a function and x is the input then we denote the corresponding output by f(x) (read “f of x”). Caution: f(x) is not a multiplication of f and x. It is an entity that can’t be split. f(x) denotes output that corresponds to the input x. Example: Let f(x) = 3x2 -2x – 4 Find f(0), f(1), f(-2), f(-x), -f(x), f(x+1), f(2x), f(x+h) f(0) = 3(0)2 -2(0) – 4 = -4 f(1) = 3(1)2 – 2(1) – 4 = -3 f(-2) = 3(-2)2 -2(-2)- 4= 12 f(-x)=3(-x)2-2(-x) -4 = 3x2 + 2x – 4 f(x+1) = 3(x+1)2 -2(x+1) – 4 = 3(x2 + 2x +1)-2x-2 -4 = 3x2 + 6x + 3 -2x – 6= 3x2 + 4x – 3 f(2x)= 3(2x)2 -2(2x) – 4 = 3(4x2) – 4x – 4= 12x2 – 4x – 4 f(x+h) = 3(x+h)2 -2(x+h) – 4 = 3(x2 + 2xh +h2) – 2x- 2h -4 = 3x2 + 6xh + 3h2 -2x – 2h - 4 -f(x)= - (3x2 – 2x – 4) = -3x2 + 2x + 4 3x 1 Example: For the function g(x) , find g(-3), g(0), g(2), g(-x), g(x-2), g(3x), g(x+h). 2 x 3(3) 1 9 1 10 Solution: g(3) 2 2 (3) 2 3 5 3(0) 1 0 1 1 g(0) 2 (0) 2 0 2 3(2) 1 6 1 5 g(2) this value is not defined; 2 does not belong to the domain of g 2 (2) 2 2 0 3(x) 1 3x 1 g(x) 2 (x) 2 x 3(x 2) 1 3x 6 1 3x 7 g(x 2) 2 (x 2) 2 x 2 4 x 3(3x) 1 9x 1 g(3x) 2 (3x) 2 3x 3(x h) 1 3x 3h 1 g(x h) 2 (x h) 2 x 3 Simplifying the difference quotient f (x h) f (x) h Example : Find (the difference quotient for f) for f(x) = 3x2 -2x - 4 f (x h) f (x) 3(x h) 2 2(x h) 4 (3x 2 2x 4) 3(x 2 2xh h 2 ) 2x 2h 4 3x 2 2x 4 h h h 3x 2 6xh 3h 2 2h 3x 2 6xh 3h 2 2h h6x 3h 2 6x 3h 2 h h h Example : g(x h) g(x) 3 Find (the difference quotient for g) for g(x) h 2 x Solution: 3 3 3 3 g(x h) g(x) 2 (x h) 2 x 2 x h 2 x 1 3(2 x) 3(2 x h) h h h h (2 x h)(2 x) 1 6 3x 6 3x 3h 1 3h 3 h (2 x h)(2 x) h (2 x h)(2 x) (2 x h)(2 x) Domains of functions When a function f is given by a formula and its domain is not given, then it is assumed that the domain is the largest set of real numbers for which f (x) can be computed and is a real number. Often it is more practical to ask what values of x cannot be used in a formula. At this point, to find a domain of a function you should examine the formula for the function and ask two questions A) Does the formula have a square root (or even index radical)? IF yes, we have to make sure that the expression inside the radical ( called radicand) is not negative. For example, if f (x) 3x 1 then we cannot allow 3x-1 to be negative, as the square root of a negative number does not exist. and B) Does the formula contain a variable in the denominator? If yes, we have to guarantee that the denominator does not become 0. For example, if 2x 2 3 f (x) , then we can allow any value for x as long x2 +2x -1 is not zero since the x 2 2x 1 division by zero is not defined. If the formula contains a variable in the denominator, the domain consists of all real numbers except the ones that make the denominator zero.
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