Problem 21 - A Graphical Approach to College Algebra

Question

Find the slope-intercept form of the equation of the line satisfying the given conditions. Do not use a calculator. Through \((0,5)\) and \((10,0)\)

Step-by-Step Solution

Answer

The equation of the line is \( y = -\frac{1}{2}x + 5 \).

Step 1: Identify two points

The given points are \( (0,5) \) and \( (10,0) \). These points lie on the line for which we want to find the equation in slope-intercept form.

Step 2: Calculate the slope

The formula for the slope \( m \) is \( m = \frac{y_2 - y_1}{x_2 - x_1} \). Substituting the given points, \( m = \frac{0 - 5}{10 - 0} = \frac{-5}{10} = -\frac{1}{2} \).

Step 3: Use point-slope form

The point-slope form of a line is \( y - y_1 = m(x - x_1) \). Using the point \( (0,5) \) and the slope \( m = -\frac{1}{2} \), the equation becomes \( y - 5 = -\frac{1}{2}(x - 0) \).

Step 4: Convert to slope-intercept form

Simplify the equation from Step 3 to get the slope-intercept form, which is \( y = mx + b \). Start by expanding: \( y - 5 = -\frac{1}{2}x \). Add 5 to both sides to isolate \( y \): \( y = -\frac{1}{2}x + 5 \).

Key Concepts

Linear Equations

Linear equations are mathematical expressions that represent straight lines on a coordinate plane. They show a direct relationship between two variables, typically represented as \( x \) and \( y \). The general form of a linear equation is \( y = mx + b \), where \( m \) is the slope of the line and \( b \) is the y-intercept. The y-intercept is the point where the line crosses the y-axis, and it is the value of \( y \) when \( x = 0 \). Linear equations are fundamental in algebra because they model real-world phenomena where a change in one variable causes a proportional change in another. To find a specific linear equation, you need two bits of information: the slope and a point through which the line passes. Understanding linear equations is crucial as they serve as a basis for more complex algebraic and geometric concepts.

Calculating Slope

The slope of a line is a measure of its steepness and is calculated using the formula \( m = \frac{y_2 - y_1}{x_2 - x_1} \). This formula requires two points on the line, represented as \( (x_1, y_1) \) and \( (x_2, y_2) \). The slope, \( m \), tells us how much \( y \) changes for a unit change in \( x \). A positive slope means the line rises as you move from left to right, while a negative slope means the line falls. If the slope is zero, the line is horizontal, indicating no change in \( y \) as \( x \) changes. In the original exercise, we used the points \((0,5)\) and \((10,0)\) to find the slope. Substituting them into the formula, we calculated \( m = -\frac{1}{2} \), showing a downward trend. Understanding how to calculate the slope is essential for transforming an equation into the slope-intercept form, allowing us to interpret the behavior of linear relationships.

Point-Slope Form

The point-slope form of a linear equation is expressed as \( y - y_1 = m(x - x_1) \). This form is particularly useful when you're given a point on a line and the slope, allowing you to write the equation with these data. Let's break it down:

• \( m \) is the slope of the line.
• \( (x_1, y_1) \) is a known point on the line.

This form can easily be converted into the more commonly used slope-intercept form. In the exercise, we started with the point-slope form using the point \((0,5)\) and slope \( -\frac{1}{2} \), resulting in the equation \( y - 5 = -\frac{1}{2}(x - 0) \). Simplifying this gives us the slope-intercept form: \( y = -\frac{1}{2}x + 5 \). Knowing how to use the point-slope form is invaluable for quickly finding and expressing the equation of a line in various formats.