Which Statement Is True About The Given Function

Here's a comprehensive article exploring how to determine the truthfulness of statements about a given function, covering various function types, analysis methods, and examples.

Determining the Truth of Statements About a Given Function

In mathematics, a function is a fundamental concept that describes a relationship between inputs and outputs. Given a function, we often need to evaluate statements made about its properties, behavior, or characteristics. This involves using a variety of analytical tools and techniques to verify whether these statements are true or false. This article provides a comprehensive guide on how to assess the validity of statements about functions, covering different types of functions, common properties, and practical methods for verification.

Understanding Functions: A Foundation

Before diving into the specifics of verifying statements, it's crucial to have a solid understanding of what functions are and the different forms they can take.

A function f from a set A to a set B is a rule that assigns each element x in A to exactly one element y in B. We write this as f(x) = y. Here:

• A is the domain of the function (the set of all possible inputs).
• B is the codomain of the function (the set containing all possible outputs).
• The range of the function is the subset of B consisting of all actual outputs of the function.

Types of Functions

Functions come in many forms, each with its own unique properties:

• Linear Functions: Functions of the form f(x) = mx + b, where m and b are constants.
• Quadratic Functions: Functions of the form f(x) = ax² + bx + c, where a, b, and c are constants.
• Polynomial Functions: Functions of the form f(x) = aₙxⁿ + aₙ₋₁xⁿ⁻¹ + ... + a₁x + a₀, where aᵢ are constants and n is a non-negative integer.
• Rational Functions: Functions of the form f(x) = P(x) / Q(x), where P(x) and Q(x) are polynomial functions.
• Trigonometric Functions: Functions like sin(x), cos(x), tan(x), etc., which relate angles of a triangle to ratios of its sides.
• Exponential Functions: Functions of the form f(x) = aˣ, where a is a constant.
• Logarithmic Functions: Functions of the form f(x) = logₐ(x), which are the inverse of exponential functions.
• Piecewise Functions: Functions defined by different expressions on different intervals of their domain.

Common Properties of Functions

Statements about functions often relate to specific properties they may possess. Understanding these properties is key to verifying the truth of such statements.

• Domain and Range: The domain is the set of all possible input values for which the function is defined, while the range is the set of all possible output values.
• Intercepts: The points where the function's graph intersects the x-axis (x-intercepts or roots) and the y-axis (y-intercept).
• Symmetry:
  • Even Functions: Functions that satisfy f(x) = f(-x). Their graphs are symmetric with respect to the y-axis.
  • Odd Functions: Functions that satisfy f(x) = -f(-x). Their graphs are symmetric with respect to the origin.
• Periodicity: A function f(x) is periodic if there exists a non-zero constant P such that f(x + P) = f(x) for all x in the domain. The smallest such P is called the period.
• Continuity: A function is continuous if its graph can be drawn without lifting the pen, meaning there are no breaks, jumps, or holes.
• Differentiability: A function is differentiable at a point if its derivative exists at that point. This implies the function is smooth and has a well-defined tangent line.
• Increasing and Decreasing Intervals: Intervals where the function's values are increasing or decreasing as x increases.
• Local Maxima and Minima: Points where the function attains a local maximum or minimum value within a specific interval.
• Global Maxima and Minima: The absolute maximum and minimum values of the function over its entire domain.
• Asymptotes: Lines that the function's graph approaches as x approaches infinity or negative infinity (horizontal asymptotes) or as x approaches a specific value (vertical asymptotes).
• Invertibility: A function is invertible if it is a one-to-one correspondence, meaning each output corresponds to exactly one input. In this case, there exists an inverse function f⁻¹(y) such that f⁻¹(f(x)) = x and f(f⁻¹(y)) = y.

Methods for Verifying Statements About Functions

To determine whether a statement about a function is true or false, we can use a variety of methods. The specific approach depends on the type of function and the nature of the statement.

1. Direct Substitution

This is a straightforward method used to check statements that involve specific values of the function.

• Procedure: Substitute the given value(s) into the function and evaluate.

• Example:

  • Function: f(x) = x² + 3x - 2
  • Statement: f(2) = 8
  • Verification: f(2) = (2)² + 3(2) - 2 = 4 + 6 - 2 = 8. The statement is true.

2. Algebraic Manipulation

This involves using algebraic techniques to simplify expressions or solve equations related to the function.

• Procedure: Apply algebraic rules and identities to manipulate the function's expression and check if it satisfies the given condition.

• Example:

  • Function: f(x) = (x + 1)² - x²
  • Statement: f(x) = 2x + 1
  • Verification: f(x) = (x² + 2x + 1) - x² = 2x + 1. The statement is true.

3. Graphical Analysis

Visualizing the function's graph can provide valuable insights into its behavior and properties.

• Procedure: Plot the function's graph and visually inspect for characteristics such as intercepts, symmetry, increasing/decreasing intervals, and asymptotes.

• Example:

  • Function: f(x) = sin(x)
  • Statement: f(x) is symmetric with respect to the origin (odd function).
  • Verification: By plotting the graph of sin(x), we can see that it is indeed symmetric with respect to the origin, confirming the statement.

4. Calculus Techniques

Calculus provides powerful tools for analyzing functions, including finding derivatives, integrals, and limits.

• Derivatives: Used to find the slope of a function, identify critical points (where the derivative is zero or undefined), and determine increasing/decreasing intervals and local maxima/minima.

• Integrals: Used to find the area under a curve and determine the average value of a function.

• Limits: Used to analyze the behavior of a function as x approaches a specific value or infinity.

  • Example:

    • Function: f(x) = x³ - 6x² + 5
    • Statement: f(x) has a local minimum at x = 4.
    • Verification:
      1. Find the derivative: f'(x) = 3x² - 12x.
      2. Set the derivative to zero: 3x² - 12x = 0 => 3x(x - 4) = 0 => x = 0, 4.
      3. Find the second derivative: f''(x) = 6x - 12.
      4. Evaluate the second derivative at x = 4: f''(4) = 6(4) - 12 = 12 > 0. Since the second derivative is positive, x = 4 corresponds to a local minimum. The statement is true.

5. Counterexamples

To prove that a statement is false, it is sufficient to find a single counterexample – a specific case that violates the statement.

• Procedure: Identify a value or scenario where the statement does not hold true.

• Example:

  • Function: f(x) = x²
  • Statement: f(x) is an increasing function for all x.
  • Verification: Consider x = -1 and x = 0. We have f(-1) = 1 and f(0) = 0. Since f(-1) > f(0), the function is decreasing on the interval (-∞, 0). Therefore, the statement is false (the counterexample disproves it).

6. Proof by Induction

This technique is used to prove statements that hold for all natural numbers.

• Procedure:

  1. Base Case: Show that the statement is true for the smallest natural number (usually n = 1).
  2. Inductive Hypothesis: Assume that the statement is true for some arbitrary natural number k.
  3. Inductive Step: Prove that the statement is true for k + 1, assuming it is true for k.

• Example:

  • Statement: The sum of the first n natural numbers is n(n + 1) / 2. That is, 1 + 2 + ... + n = n(n + 1) / 2.
  • Verification:

    1. Base Case: For n = 1, 1 = 1(1 + 1) / 2 = 1. The statement is true.

    2. Inductive Hypothesis: Assume 1 + 2 + ... + k = k(k + 1) / 2.

    3. Inductive Step: We need to show that 1 + 2 + ... + (k + 1) = (k + 1)(k + 2) / 2.

       • Starting with the left side: 1 + 2 + ... + (k + 1) = (1 + 2 + ... + k) + (k + 1)
       • Using the inductive hypothesis: k(k + 1) / 2 + (k + 1) = [k(k + 1) + 2(k + 1)] / 2 = (k + 1)(k + 2) / 2
       • This matches the right side of the equation, so the statement is true for k + 1.

By the principle of mathematical induction, the statement is true for all natural numbers n.

Practical Examples and Applications

Let's explore some practical examples of verifying statements about functions.

Example 1: Analyzing a Quadratic Function

• Function: f(x) = -x² + 4x - 3

• Statement 1: f(x) has a maximum value.

• Statement 2: The maximum value of f(x) occurs at x = 2.

• Statement 3: f(x) > 0 for all x.

• Verification:

  1. Statement 1: Since the coefficient of x² is negative, the parabola opens downward, indicating that the function has a maximum value. The statement is true.
  2. Statement 2: To find the x-coordinate of the vertex (where the maximum occurs), we use the formula x = -b / (2a) = -4 / (2 * -1) = 2. The statement is true.
  3. Statement 3: To check this, we can find the roots of the quadratic equation: -x² + 4x - 3 = 0. Factoring gives us -(x - 1)(x - 3) = 0, so the roots are x = 1 and x = 3. Since the parabola opens downward, f(x) > 0 only between the roots (i.e., 1 < x < 3). Therefore, the statement is false.

Example 2: Analyzing a Trigonometric Function

• Function: f(x) = 2cos(3x)

• Statement 1: The amplitude of f(x) is 2.

• Statement 2: The period of f(x) is 2π.

• Statement 3: f(x) is an even function.

• Verification:

  1. Statement 1: The amplitude of a function Acos(Bx) is |A|. In this case, A = 2, so the amplitude is |2| = 2. The statement is true.

  2. Statement 2: The period of a function Acos(Bx) is 2π / |B|. In this case, B = 3, so the period is 2π / 3. Therefore, the statement is false.

  3. Statement 3: To check if f(x) is even, we need to see if f(x) = f(-x).

     • f(-x) = 2cos(3(-x)) = 2cos(-3x)
     • Since cos(-θ) = cos(θ), we have 2cos(-3x) = 2cos(3x) = f(x).
     • Therefore, f(x) is an even function, and the statement is true.

Example 3: Analyzing a Rational Function

• Function: f(x) = (x + 1) / (x - 2)

• Statement 1: f(x) has a vertical asymptote at x = 2.

• Statement 2: f(x) has a horizontal asymptote at y = 1.

• Statement 3: f(0) = -1/2.

• Verification:

  1. Statement 1: A rational function has a vertical asymptote where the denominator is zero. The denominator x - 2 = 0 when x = 2. Therefore, f(x) has a vertical asymptote at x = 2. The statement is true.

  2. Statement 2: To find the horizontal asymptote, we analyze the limit as x approaches infinity.

     • lim (x→∞) (x + 1) / (x - 2) = lim (x→∞) (1 + 1/x) / (1 - 2/x) = 1 / 1 = 1.
     • Therefore, f(x) has a horizontal asymptote at y = 1. The statement is true.

  3. Statement 3: To find f(0), we substitute x = 0 into the function.

     • f(0) = (0 + 1) / (0 - 2) = 1 / -2 = -1/2. The statement is true.

Advanced Techniques and Considerations

In more complex scenarios, verifying statements about functions may require advanced techniques and considerations.

• Complex Analysis: For complex-valued functions, complex analysis techniques such as Cauchy's integral formula and the residue theorem may be necessary.
• Functional Analysis: In the context of infinite-dimensional spaces, functional analysis provides tools for studying functions as elements of vector spaces.
• Numerical Methods: When analytical solutions are not possible, numerical methods can be used to approximate function values and properties.
• Computer Algebra Systems (CAS): Tools like Mathematica, Maple, and SymPy can be used to perform symbolic calculations, plot graphs, and analyze functions.

Common Mistakes to Avoid

When verifying statements about functions, it's important to avoid common pitfalls:

• Assuming Properties: Do not assume that a function has certain properties without verifying them.
• Incorrectly Applying Formulas: Ensure that formulas and techniques are applied correctly and appropriately.
• Ignoring Domain Restrictions: Pay attention to the domain of the function, as certain operations may not be valid for all values of x.
• Overgeneralizing from Examples: A few examples are not sufficient to prove a statement; a general proof is required.
• Confusing Correlation with Causation: Just because two functions behave similarly does not mean they are related in a specific way.

Conclusion

Verifying statements about functions requires a combination of understanding function properties, applying appropriate analytical techniques, and careful attention to detail. Whether using direct substitution, algebraic manipulation, graphical analysis, calculus techniques, or proof by induction, the key is to approach each statement systematically and rigorously. By mastering these methods, you can confidently determine the truth of statements about any given function, enhancing your understanding and problem-solving skills in mathematics and related fields.