SAT Math : How to find the equation of a line

Study concepts, example questions & explanations for SAT Math

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Example Questions

Example Question #524 : Sat Mathematics

 \(\displaystyle \small \textup{Which is the equation of the line that passes through the points}\)

\(\displaystyle \small (1,2)\,\textup{and}\,(3.8)?\)

Possible Answers:

\(\displaystyle \small y = x+5\)

\(\displaystyle \small y=\frac{x}{3}+1\)

\(\displaystyle \small y=3x+2\)

\(\displaystyle \small y=3x-1\)

\(\displaystyle \small y=-3x +2\)

Correct answer:

\(\displaystyle \small y=3x-1\)

Explanation:

Equation of line: \(\displaystyle \small y = mx+b\),   \(\displaystyle \small m\) = slope, \(\displaystyle \small b\) = \(\displaystyle y\)-intercept

Step 1) Find slope (\(\displaystyle \small m\)):  rise/run    \(\displaystyle \small \frac{8-2}{3-1}=\frac{6}{2}=3\)

Step 2) Find \(\displaystyle y\)-intercept (\(\displaystyle \small b\)):   \(\displaystyle \small \frac{2-b}{1-0}=3\) 

                                                      \(\displaystyle \small 2-b = 3\)

                                                      \(\displaystyle \small 2 = 3 + b\)

                                                       \(\displaystyle \small -1 =b\)

\(\displaystyle \small y=3x-1\)

Example Question #525 : Sat Mathematics

Whast line goes through the points \(\displaystyle (1,3)\) and \(\displaystyle (7,5)\)?

Possible Answers:

\(\displaystyle x+y=4\)

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

\(\displaystyle -x+3y=8\)

\(\displaystyle 3x-2y=9\)

\(\displaystyle x-2y=6\)

Correct answer:

\(\displaystyle -x+3y=8\)

Explanation:

Let P_{1}=(1,3)\(\displaystyle P_{1}=(1,3)\) and P_{2}=(7,5)\(\displaystyle P_{2}=(7,5)\)

The slope is geven by:  m = (y_{2} - y_{1}) \div (x_{2} - x_{1})\(\displaystyle m = (y_{2} - y_{1}) \div (x_{2} - x_{1})\)  so

\(\displaystyle m=\frac{1}{3}\)

Then we use the slope-intercept form of an equation;  \(\displaystyle y=mx+b\) so

\(\displaystyle b=\frac{8}{3}\)

And we convert 

\(\displaystyle y=\frac{1}{3}x+\frac{8}{3}\) 

to standard form.

Example Question #21 : Psat Mathematics

What is the equation of the line that passes through the points (4,7) and (8,10)?

Possible Answers:

\(\displaystyle 3x-4y=16\)

\(\displaystyle 3x-4y=-16\)

\(\displaystyle 3x+4y=16\)

\(\displaystyle -3x-4y=16\)

\(\displaystyle 3x+4y=-16\)

Correct answer:

\(\displaystyle 3x-4y=-16\)

Explanation:

In order to find the equation of the line, we will first need to find the slope between the two points through which it passes. The slope, \(\displaystyle m\), of a line that passes through the points \(\displaystyle (x_1,y_1)\) and \(\displaystyle (x_2,y_2)\) is given by the formula below:

\(\displaystyle \text{slope}=m=\frac{y_2-y_1}{x_2-x_1}\)

We are given our two points, (4,7) and (8,10), allowing us to calculate the slope.

\(\displaystyle m=\frac{10-7}{8-4}=\frac{3}{4}\)

Next, we can use point slope form to find the equation of a line with this slope that passes through one of the given points. We will use (4,7).

\(\displaystyle y-y_1=m(x-x_1)\)

\(\displaystyle y-7=\frac{3}{4}(x-4)\)

Multiply both sides by four to eliminate the fraction, and simplify by distribution.

\(\displaystyle 4(y-7)=4(\frac{3}{4})(x-4)\)

\(\displaystyle 4y-28=3(x-4)\)

\(\displaystyle 4y-28=3x-12\)

Subtract \(\displaystyle 4y\) from both sides and add twelve to both sides.

\(\displaystyle -28=3x-4y-12\)

\(\displaystyle -16=3x-4y\)

This gives our final answer: \(\displaystyle 3x-4y=-16\)

Example Question #22 : Psat Mathematics

Which line contains the following ordered pairs:

\(\displaystyle \left ( -2,4 \right )\) and \(\displaystyle \left ( 6,2 \right )\)

Possible Answers:

\small y=-\frac{1}{4}x+\frac{7}{2}\(\displaystyle \small y=-\frac{1}{4}x+\frac{7}{2}\)

\small y=-x+14\(\displaystyle \small y=-x+14\)

\small y=\frac{1}{4}x+\frac{7}{2}\(\displaystyle \small y=\frac{1}{4}x+\frac{7}{2}\)

\small y=x+14\(\displaystyle \small y=x+14\)

Correct answer:

\small y=-\frac{1}{4}x+\frac{7}{2}\(\displaystyle \small y=-\frac{1}{4}x+\frac{7}{2}\)

Explanation:

First, solve for slope.

\small m=\frac{\Delta y}{\Delta x}=\frac{2-4}{6-(-2)}=\frac{-2}{8}=-\frac{1}{4}\(\displaystyle \small m=\frac{\Delta y}{\Delta x}=\frac{2-4}{6-(-2)}=\frac{-2}{8}=-\frac{1}{4}\)

Then, substitute one of the points into the equation y=mx+b.

\small 2=(-\frac{1}{4})(6)+b\(\displaystyle \small 2=(-\frac{1}{4})(6)+b\)

\small 2=(-\frac{3}{2})+b\(\displaystyle \small 2=(-\frac{3}{2})+b\)

\small b=2+\frac{3}{2}=\frac{7}{2}\(\displaystyle \small b=2+\frac{3}{2}=\frac{7}{2}\)

This leaves us with the equation \small y=-\frac{1}{4}+\frac{7}{2}\(\displaystyle \small y=-\frac{1}{4}+\frac{7}{2}\)

Example Question #521 : Geometry

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

 

Act_math_160_04

Possible Answers:

\(\displaystyle y=-x\)

\(\displaystyle y=-5x-4\)

\(\displaystyle y=\frac{10}{3}x-4\)

\(\displaystyle y=x-4\)

Correct answer:

\(\displaystyle y=\frac{10}{3}x-4\)

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.

 

Example Question #524 : Geometry

Which line passes through the points (0, 6) and (4, 0)?

Possible Answers:

y = 2/3 + 5

y = –3/2 – 3

y = 2/3x –6

y = –3/2x + 6

y = 1/5x + 3

Correct answer:

y = –3/2x + 6

Explanation:

P1 (0, 6) and P2 (4, 0)

First, calculate the slope:  m = rise ÷ run = (y2 – y1)/(x– x1), so m = –3/2

Second, plug the slope and one point into the slope-intercept formula: 

y = mx + b, so 0 = –3/2(4) + b and b = 6

Thus, y = –3/2x + 6

Example Question #62 : Coordinate Geometry

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

Possible Answers:

4x – 5y = 4

2x – 3y = 5

–2x + 2y = 3

3x + 5y = 2

–3x + 2y = 3

Correct answer:

–3x + 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.

Example Question #1 : Coordinate Geometry

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

Possible Answers:

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

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

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

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

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

Correct answer:

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

Explanation:

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

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

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

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

Example Question #63 : Coordinate Geometry

A line is defined by the following equation:

\(\displaystyle 7x+28y=84\)

What is the slope of that line?

Possible Answers:

\(\displaystyle 4\)

\(\displaystyle -4\)

\(\displaystyle \frac{1}{4}\)

\(\displaystyle -\frac{1}{4}\)

\(\displaystyle 28\)

Correct answer:

\(\displaystyle -\frac{1}{4}\)

Explanation:

The equation of a line is

y=mx + b where m is the slope

Rearrange the equation to match this:

7x + 28y = 84

28y = -7x + 84

y = -(7/28)x + 84/28

y = -(1/4)x + 3

m = -1/4

Example Question #1 : Lines

If the coordinates (3, 14) and (5, 15) are on the same line, what is the equation of the line?

Possible Answers:

\(\displaystyle y=-8x+38\)

\(\displaystyle y=\frac{1}{8}x+14.375\)

\(\displaystyle y=-\frac{1}{8}x+13.625\)

\(\displaystyle y=-8x-38\)

\(\displaystyle y=-\frac{1}{8}x+14.375\)

Correct answer:

\(\displaystyle y=-\frac{1}{8}x+14.375\)

Explanation:

First solve for the slope of the line, m using y=mx+b

m = (y2 – y1) / (x2 – x1)

= (15  14) / (5 3)

= (1 )/( 8)

=1/8

y = (1/8)x + b

Now, choose one of the coordinates and solve for b:

14 = (1/8)3 + b

14 = 3/8 + b

b = 14 + (3/8)

b = 14.375

y = (1/8)x + 14.375

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