2Chapter Outline The Slope of a Straight Line The Slope of a Curve at a PointThe DerivativeLimits and the DerivativeDifferentiability and ContinuitySome Rules for DifferentiationMore About DerivativesThe Derivative as a Rate of Change
4Section Outline Nonvertical Lines Positive and Negative Slopes of LinesInterpretation of a GraphProperties of the Slope of a Nonvertical LineFinding the Slope and y-Intercept of a LineSketching the Graph of a LineMaking Equations of LinesSlope as a Rate of Change
5Nonvertical Lines Definition Example Equations of Nonvertical Lines: A nonvertical line L has an equation of the formThe number m is called the slope of L and the point (0, b) is called the y-intercept. The equation above is called the slope-intercept equation of L.For this line, m = 3 and b = -4.
6Lines – Positive Slope EXAMPLE The following are graphs of equations of lines that have positive slopes.
7Lines – Negative Slope EXAMPLE The following are graphs of equations of lines that have negative slopes.
8Interpretation of a Graph EXAMPLEA salesperson’s weekly pay depends on the volume of sales. If she sells x units of goods, then her pay is y = 5x + 60 dollars. Give an interpretation of the slope and the y-intercept of this straight line.SOLUTIONFirst, let’s graph the line to help us understand the exercise.(80, 460)
9Interpretation of a Graph CONTINUEDThe slope is 5, or 5/1. Since the numerator of this fraction represents the amount of change in her pay relative to the amount of change in her sales, the denominator, for every 1 sale that she makes, her pay increases by 5 dollars.The y-intercept is 60 and occurs on the graph at the point (0, 60). This point suggests that when she has executed 0 sales, her pay is 60 dollars. This $60 could be referred to as her base pay.
12Finding Slope and y-intercept of a Line EXAMPLEFind the slope and y-intercept of the lineSOLUTIONFirst, we write the equation in slope-intercept form.This is the given equation.Divide both terms of the numerator of the right side by 3.RewriteSince the number being multiplied by x is 1/3, 1/3 is the slope of the line. Since the other 1/3 is the number being added to the term containing x, 1/3, or (0, 1/3), is the y-intercept.Incidentally, it was a complete coincidence that the slope and y-intercept were the same number. This does not normally occur.
13Sketching Graphs of Lines EXAMPLESketch the graph of the line passing through (-1, 1) with slope ½.SOLUTIONWe use Slope Property 1. We begin at the given point (-1, 1) and from there, move up one unit and to the right two units to find another point on the line.(-1, 1)(-1, 1)
14Sketching Graphs of Lines CONTINUEDNow we connect the two points that have already been determined, since two points determine a straight line.
15Making Equations of Lines EXAMPLEFind an equation of the line that passes through the points (-1/2, 0) and (1, 2).SOLUTIONTo find an equation of the line that passes through those two points, we need a point (we already have two) and a slope. We do not yet have the slope so we must find it. Using the two points we will determine the slope by using Slope Property 2.We now use Slope Property 3 to find an equation of the line. To use this property we need the slope and a point. We can use either of the two points that were initially provided. We’ll use the second (the first would work just as well).
16Making Equations of Lines CONTINUEDThis is the equation from Property 3.(x1, y1) = (1, 2) and m = 4/3.Distribute.Add 2 to both sides of the equation.NOTE: Technically, we could have stopped when the equation looked like since it is an equation that represents the line and is equivalent to our final equation.
17Making Equations of Lines EXAMPLEFind an equation of the line that passes through the point (2, 0) and is perpendicular to the line y = 2x.SOLUTIONTo find an equation of the line, we need a point (we already have one) and a slope. We do not yet have the slope so we must find it. We know that the line we desire is perpendicular to the line y = 2x. Using Slope Property 5, we know that the product of the slope of the line desired and the slope of the line y = 2x is -1. We recognize that the line y = 2x is in slope-intercept form and therefore the slope of the line is 2. We can now find the slope of the line that we desire. Let the slope of the new line be m.(slope of a line)(slope of a new line) = -1This is Property 5.2m = -1The slope of one line is 2 and the slope of the desired line is denoted by m.m = -0.5Divide.Now we can find the equation of the desired line using Property 3.
18Making Equations of Lines CONTINUEDThis is the equation from Property 3.(x1, y1) = (2, 0) and m = -0.5.Distribute.
19Slope as a Rate of Change EXAMPLECompute the rate of change of the function over the given intervals.SOLUTIONWe first get y by itself in the given equation.This is the given equation.Subtract 2x from both sides.Since this is clearly a linear function (since it’s now in slope-intercept form) it has constant slope, namely -2. Therefore, by definition, it also has a constant rate of change, -2. Therefore, no matter what interval is considered for this function, the rate of change will be -2. Therefore the answer, for both intervals, is -2.