How to find the equation of a line

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GRE Quantitative Reasoning › How to find the equation of a line

Questions 1 - 10
1

What is the equation of the line passing through (–1,5) and the upper-right corner of a square with a center at the origin and a perimeter of 22?

y = (3/5)x + 22/5

y = (–3/5)x + 22/5

y = (–3/5)x + 28/5

y = (–1/5)x + 2.75

y = –x + 5

Explanation

If the square has a perimeter of 22, each side is 22/4 or 5.5. This means that the upper-right corner is (2.75, 2.75)—remember that each side will be "split in half" by the x and y axes.

Using the two points we have, we can ascertain our line's equation by using the point-slope formula. Let us first get our slope:

m = rise/run = (2.75 – 5)/(2.75 + 1) = –2.25/3.75 = –(9/4)/(15/4) = –9/15 = –3/5.

The point-slope form is: y – y0 = m(x – x0). Based on our data this is: y – 5 = (–3/5)(x + 1); Simplifying, we get: y = (–3/5)x – (3/5) + 5; y = (–3/5)x + 22/5

2

Let y = 3_x_ – 6.

At what point does the line above intersect the following:

They do not intersect

They intersect at all points

(0,–1)

(–5,6)

(–3,–3)

Explanation

If we rearrange the second equation it is the same as the first equation. They are the same line.

3

What line goes through the points (1, 3) and (3, 6)?

3x + 5y = 2

2x – 3y = 5

4x – 5y = 4

–3x + 2y = 3

–2x + 2y = 3

Explanation

If P1(1, 3) and P2(3, 6), then calculate the slope by m = rise/run = (y2 – y1)/(x2 – x1) = 3/2

Use the slope and one point to calculate the intercept using y = mx + b

Then convert the slope-intercept form into standard form.

4

What is the equation of a line passing through with a -intercept of ?

Explanation

Based on the information that you have been provided, you can quickly find the slope of your line. Since the y-intercept is , you know that the line contains the point . Therefore, the slope of the line is found:

Based on this information, you can use the standard slope-intercept form to find your equation:

, where and

5

What is the slope-intercept form of \dpi{100} \small 8x-2y-12=0?

\dpi{100} \small y=4x-6

\dpi{100} \small y=4x+6

\dpi{100} \small y=2x-3

\dpi{100} \small y=-4x+6

\dpi{100} \small y=-2x+3

Explanation

The slope intercept form states that \dpi{100} \small y=mx+b. In order to convert the equation to the slope intercept form, isolate \dpi{100} \small y on the left side:

\dpi{100} \small 8x-2y=12

\dpi{100} \small -2y=-8x+12

\dpi{100} \small y=4x-6

6

Which of the following equations does NOT represent a line?

Explanation

The answer is .

A line can only be represented in the form or , for appropriate constants , , and . A graph must have an equation that can be put into one of these forms to be a line.

represents a parabola, not a line. Lines will never contain an term.

7

Given the graph of the line below, find the equation of the line.

Act_math_160_04

Explanation

To solve this question, you could use two points such as (1.2,0) and (0,-4) to calculate the slope which is 10/3 and then read the y-intercept off the graph, which is -4.

8

What is the equation of the straight line passing through (–2, 5) with an x-intercept of 3?

y = –x + 3

y = –5x + 3

y = –x – 3

y = –5x – 3

Explanation

First you must figure out what point has an x-intercept of 3. This means the line crosses the x-axis at 3 and has no rise or fall on the y-axis which is equivalent to (3, 0). Now you use the formula (y2 – y1)/(x2 – x1) to determine the slope of the line which is (5 – 0)/(–2 – 3) or –1. Now substitute a point known on the line (such as (–2, 5) or (3, 0)) to determine the y-intercept of the equation y = –x + b. b = 3 so the entire equation is y = –x + 3.

Gre_quant_179_01

9

What is the equation of a line passing through the two points and ?

Explanation

Based on the information provided, you can find the slope of this line easily. From that, you can use the point-slope form of the equation of a line to compute the line's full equation. The slope is merely:

Now, for a point and a slope , the point-slope form of a line is:

Let's use for our point

This gives us:

Now, distribute and solve for :

10

What is the equation of a line passing through the points and ?

Explanation

Based on the information provided, you can find the slope of this line easily. From that, you can use the point-slope form of the equation of a line to compute the line's full equation. The slope is merely:

Now, for a point and a slope , the point-slope form of a line is:

Let's use for our point

This gives us:

Now, distribute and solve for :

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