Math Problem Statement

The table shows three values of x and their corresponding values of g(x), where g(x) = f(x) / (x + 5) and f is a linear function. What is the y-intercept of the graph of y = f(x) in the xy-plane?

Solution

To solve the problem in the image, we need to focus on determining the y-intercept of the function y=f(x)y = f(x) given the relationship g(x)=f(x)x+5g(x) = \frac{f(x)}{x + 5}, and the table with corresponding values of xx and g(x)g(x).

Step-by-step breakdown:

  1. Identify the relationship between f(x)f(x) and g(x)g(x): g(x)=f(x)x+5g(x) = \frac{f(x)}{x + 5} From this, we can rearrange the equation to express f(x)f(x) in terms of g(x)g(x) and xx: f(x)=g(x)(x+5)f(x) = g(x)(x + 5)

  2. Use the values from the table: Let's compute f(x)f(x) for each xx and g(x)g(x) value from the table:

    • For x=20x = -20, g(20)=4g(-20) = 4: f(20)=4×(20+5)=4×(15)=60f(-20) = 4 \times (-20 + 5) = 4 \times (-15) = -60
    • For x=8x = -8, g(8)=0g(-8) = 0: f(8)=0×(8+5)=0f(-8) = 0 \times (-8 + 5) = 0
    • For x=10x = 10, g(10)=6g(10) = 6: f(10)=6×(10+5)=6×15=90f(10) = 6 \times (10 + 5) = 6 \times 15 = 90

    So, the points we have for f(x)f(x) are:

    • (20,60)(-20, -60)
    • (8,0)(-8, 0)
    • (10,90)(10, 90)
  3. Find the equation of the linear function f(x)f(x): We now have two points that we can use to find the slope mm. Using the points (8,0)(-8, 0) and (10,90)(10, 90), the slope is: m=90010(8)=9018=5m = \frac{90 - 0}{10 - (-8)} = \frac{90}{18} = 5

    Now that we have the slope m=5m = 5, we can use the point-slope form of the equation f(x)=mx+bf(x) = mx + b, where bb is the y-intercept. Using the point (8,0)(-8, 0): 0=5(8)+b0 = 5(-8) + b 0=40+b0 = -40 + b b=40b = 40

    Thus, the equation of the line is: f(x)=5x+40f(x) = 5x + 40

  4. Determine the y-intercept: The y-intercept is the value of f(x)f(x) when x=0x = 0: f(0)=5(0)+40=40f(0) = 5(0) + 40 = 40

So, the y-intercept of the graph of y=f(x)y = f(x) is (0,40)(0, 40).

Final Answer:

The correct choice is D (0,40)(0, 40).


Would you like further details on any part of this solution?

Here are 5 questions to expand your understanding:

  1. How would the solution change if f(x)f(x) were quadratic instead of linear?
  2. Can we derive the slope of a line using any two points on the line? Why?
  3. What is the significance of the y-intercept in a linear function?
  4. How can we use the point-slope form of a line to solve other types of problems?
  5. What happens to the slope of a line as the line becomes more vertical?

Tip: Whenever you are given a linear function, the y-intercept is simply the value of yy when x=0x = 0. Always solve for that first to save time.

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Math Problem Analysis

Mathematical Concepts

Linear Functions
Algebra
Slope and Intercept

Formulas

g(x) = f(x) / (x + 5)
f(x) = g(x)(x + 5)
y = mx + b

Theorems

Slope Formula
Point-Slope Form
Linear Equation

Suitable Grade Level

Grades 9-12