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Write the point-slope form of an equation of a line that passes through the points ( –1 , 3 )   and ( 0 , 0 ) . Then rewrite it in the slope-intercept form.

y 0 = 3 ( x 0 ) ; y = 3 x

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Writing and interpreting an equation for a linear function

Now that we have written equations for linear functions in both the slope-intercept form and the point-slope form, we can choose which method to use based on the information we are given. That information may be provided in the form of a graph, a point and a slope, two points, and so on. Look at the graph of the function f in [link] .

Graph depicting how to calculate the slope of a line

We are not given the slope of the line, but we can choose any two points on the line to find the slope. Let’s choose ( 0 ,   7 ) and ( 4 ,   4 ) . We can use these points to calculate the slope.

m = y 2 y 1 x 2 x 1    = 4 7 4 0    = 3 4

Now we can substitute the slope and the coordinates of one of the points into the point-slope form.

y y 1 = m ( x x 1 )   y 4 = 3 4 ( x 4 )

If we want to rewrite the equation in the slope-intercept form, we would find

y 4 = 3 4 ( x 4 ) y 4 = 3 4 x + 3        y = 3 4 x + 7

If we wanted to find the slope-intercept form without first writing the point-slope form, we could have recognized that the line crosses the y -axis when the output value is 7. Therefore, b = 7. We now have the initial value b and the slope m so we can substitute m and b into the slope-intercept form of a line.

So the function is f ( x ) = 3 4 x + 7 , and the linear equation would be y = 3 4 x + 7.

Given the graph of a linear function, write an equation to represent the function.

  1. Identify two points on the line.
  2. Use the two points to calculate the slope.
  3. Determine where the line crosses the y -axis to identify the y -intercept by visual inspection.
  4. Substitute the slope and y -intercept into the slope-intercept form of a line equation.

Writing an equation for a linear function

Write an equation for a linear function given a graph of f shown in [link] .

Graph of an increasing function with points at (-3, 0) and (0, 1).

Identify two points on the line, such as ( 0 ,   2 ) and ( 2 , −4 ) . Use the points to calculate the slope.

m = y 2 y 1 x 2 x 1    = 4 2 2 0    = 6 2    = 3

Substitute the slope and the coordinates of one of the points into the point-slope form.

y y 1 = m ( x x 1 ) y ( 4 ) = 3 ( x ( 2 ) ) y + 4 = 3 ( x + 2 )

We can use algebra to rewrite the equation in the slope-intercept form.

y + 4 = 3 ( x + 2 ) y + 4 = 3 x + 6        y = 3 x + 2
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Writing an equation for a linear cost function

Suppose Ben starts a company in which he incurs a fixed cost of $1,250 per month for the overhead, which includes his office rent. His production costs are $37.50 per item. Write a linear function C where C ( x ) is the cost for x items produced in a given month.

The fixed cost is present every month, $1,250. The costs that can vary include the cost to produce each item, which is $37.50 for Ben. The variable cost, called the marginal cost, is represented by 37.5. The cost Ben incurs is the sum of these two costs, represented by C ( x ) = 1250 + 37.5 x .

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Writing an equation for a linear function given two points

If f is a linear function, with f ( 3 ) = −2 , and f ( 8 ) = 1 , find an equation for the function in slope-intercept form.

We can write the given points using coordinates.

f ( 3 ) = −2 ( 3 , −2 ) f ( 8 ) = 1 ( 8 , 1 )

We can then use the points to calculate the slope.

m = y 2 y 1 x 2 x 1    = 1 ( 2 ) 8 3    = 3 5

Substitute the slope and the coordinates of one of the points into the point-slope form.

     y y 1 = m ( x x 1 ) y ( 2 ) = 3 5 ( x 3 )

We can use algebra to rewrite the equation in the slope-intercept form.

y + 2 = 3 5 ( x 3 ) y + 2 = 3 5 x 9 5        y = 3 5 x 19 5
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Questions & Answers

how can are find the domain and range of a relations
austin Reply
A cell phone company offers two plans for minutes. Plan A: $15 per month and $2 for every 300 texts. Plan B: $25 per month and $0.50 for every 100 texts. How many texts would you need to send per month for plan B to save you money?
Diddy Reply
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Robert
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How would you find if a radical function is one to one?
Peighton Reply
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SLIMANE
Thanks po.
Jenica
Hey I am new to precalculus, and wanted clarification please on what sine is as I am floored by the terms in this app? I don't mean to sound stupid but I have only completed up to college algebra.
rachel Reply
I don't know if you are looking for a deeper answer or not, but the sine of an angle in a right triangle is the length of the opposite side to the angle in question divided by the length of the hypotenuse of said triangle.
Marco
can you give me sir tips to quickly understand precalculus. Im new too in that topic. Thanks
Jenica
if you remember sine, cosine, and tangent from geometry, all the relationships are the same but they use x y and r instead (x is adjacent, y is opposite, and r is hypotenuse).
Natalie
it is better to use unit circle than triangle .triangle is only used for acute angles but you can begin with. Download any application named"unit circle" you find in it all you need. unit circle is a circle centred at origine (0;0) with radius r= 1.
SLIMANE
What is domain
johnphilip
the standard equation of the ellipse that has vertices (0,-4)&(0,4) and foci (0, -15)&(0,15) it's standard equation is x^2 + y^2/16 =1 tell my why is it only x^2? why is there no a^2?
Reena Reply
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Reena Reply
This term is plural for a focus, it is used for conic sections. For more detail or other math questions. I recommend researching on "Khan academy" or watching "The Organic Chemistry Tutor" YouTube channel.
Chris
how to determine the vertex,focus,directrix and axis of symmetry of the parabola by equations
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Den Reply
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Jeffrey
Confunction Identity
Barcenas
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meena
For each year t, the population of a forest of trees is represented by the function A(t) = 117(1.029)t. In a neighboring forest, the population of the same type of tree is represented by the function B(t) = 86(1.025)t.
Shakeena Reply
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Shakeena
32.243
Kenard
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Rhudy Reply
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Drew Reply
It's just like any other number. The important thing to know is that they exist and can be used in computations like any number.
Steve
I would like to add that they are used in AC signal analysis for one thing
Scott
Good call Scott. Also radar signals I believe.
Steve
They are used in any profession where the phase of a waveform has to be accounted for in the calculations. Imagine two electrical signals in a wire that are out of phase by 90°. At some times they will interfere constructively, others destructively. Complex numbers simplify those equations
Tim
Practice Key Terms 7

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Source:  OpenStax, Precalculus. OpenStax CNX. Jan 19, 2016 Download for free at https://legacy.cnx.org/content/col11667/1.6
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