2.4 Writing Equations of Lines p. 91. Learning Target I can write equations of a line.
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Transcript of 2.4 Writing Equations of Lines p. 91. Learning Target I can write equations of a line.
2.4 Writing Equations of Lines
p. 91
Learning Target
• I can write equations of a line.
• We’ve learned to graph given an equation.
• Now we’ll learn to write the equation given the graph
• There are three ways.• It all depends on what information
you are given as to which process you use.
Given the slope, m, and the y-intercept, b, use the equation
y=mx+b
•The y-intercept is -3•b=-3•The slope is 4/3
•The equation is:•y = 4/3x – 3
4
3
Use Point Slope Form:If you are given slope, m, and a
point (x1,y1) on the line
•y – y1 = m ( x – x1)
Write an equation of a line containing the point (1,2) with
slope of -1/2.
• Use (x1,y1) = (1,2) & m = -1/2
• y – 2 = -1/2 ( x – 1)• Now you can simplify to the slope
intercept form• y – 2 = -1/2 x + ½• y = -1/2 x + 5/2
If you are given two points on the line
• Find the slope using the two points
• Then plug this slope and either one of the points into the point slope formula.
Given two points (-2,2) & (3,7)• Find the slope: • m=1• Plug this slope and one of
the points into the point slope formula.
• y – 2 = 1 ( x – (-2))• y – 2 = x + 2• y = x + 4 (put the
equation into slope intercept form)
(-2,2)
(3,7)
Direct Variation
• 2 variables X & Y show direct variation
provided y = kx & k ≠ 0.
• The non-zero constant k is called the constant of variation, & y is said to vary directly with x.
• The graph of y=kx is a line thru the origin.
The variables x & y vary directly, & y = 12 when x = 4.
Write and graph an equation relating x & y.
• Use x & y to find the constant of variation• y = kx• 12 = k4• 3 = k• The direct variation equation is y = 3x• This is what you MUST write as the
answer!
Graph y = 3x
In the same direct variation equation, when x=5, find the
value of y.
•y = 3*5
•y = 15
ResourcesResources
http://www.taosschools.org/ths/Departments/MathDept/spitz/Courses/Algebra2/presentations.htm