Question

Find the equation of the line given (3,4) and (-1,8)

(help. i don't know what i did to get the incorrect answer. i got -1x+y=1, but it's wrong)​

Answer 1

Slope intercept form is y = -x+7

Standard form is x+y = 7

Both forms represent the same line, just written in different ways of course.

The graph is shown below.

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Step-by-step explanation:

Let's check the answer you got.

The equation -1x+y=1 is the same as -x+y = 1

Plug the coordinates of the first point into this equation

-x+y = 1

-3+4 = 1

1 = 1

The first point is on the line -x+y = 1

Repeat similar steps for the other point

-x+y = 1

-(-1)+8 = 1

1+8 = 1

9 = 1

The two sides don't match up. We need the same number on both sides to get a true equation. The false equation tells us that (-1,8) is not on the line -x+y = 1.

In short, this rules out -x+y = 1 being the answer.

The next section will show one way to find the answer.

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First we'll need the slope of the line through (3,4) and (-1,8)

`(x_1,y_1) = (3,4) \text{ and } (x_2,y_2) = (-1,8)\\\\m = \text{slope} = \frac{\text{rise}}{\text{run}} = \frac{\text{change in y}}{\text{change in x}}\\\\m = \frac{\text{y}_(2) - \text{y}_(1)}{\text{x}_(2) - \text{x}_(1)}\\\\m = (8 - 4)/(-1 - 3)\\\\m = (4)/(-4)\\\\m = -1\\\\`

Then I'll use the y = mx+b template to find b. For the (x,y), use either point given to you. I'll use (3,4).

So,

y = mx+b

4 = -1*3+b

4 = -3+b

4+3 = b

7 = b

b = 7

We found that m = -1 and b = 7.

We go from y = mx+b to y = -1x+7 aka y = -x+7 which is the answer in slope intercept form.

To convert to standard form Ax+By = C, add x to both sides. The goal is to get the variable terms to one side, and the constant isolated on its own side.

y = -x+7

y+x = -x+7+x

x+y = 7 is the answer in standard form

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Check:

Plug (x,y) = (3,4) into the equation to see if we get a true result.

x+y = 7

3+4 = 7

7 = 7

That works out. Now do the other pair of coordinates (-1,8).

x+y = 7

-1+8 = 7

7 = 7

That works as well. The answers are fully confirmed to be correct.

If you wanted, you can use the equation y = -x+7 as the way to check the answer. It's up to you which you prefer better if it's slope-intercept or standard form.

Yet another way to check the answer is to use a graphing tool like GeoGebra. It's something I use all the time. See the screenshot diagram below.