ACT Math : Coordinate Plane

Study concepts, example questions & explanations for ACT Math

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Example Questions

Example Question #1 : How To Find The Equation Of A Perpendicular Line

Line m passes through the points (1, 4) and (5, 2). If line p is perpendicular to m, then which of the following could represent the equation for p?

Possible Answers:

2x  y = 3

2x + y = 3

4x  3y = 4

3x + 2y = 4

x  y = 3

Correct answer:

2x  y = 3

Explanation:

The slope of m is equal to   y2-y1/x2-x1  =  2-4/5-1 -1/2                                  

Since line p is perpendicular to line m, this means that the products of the slopes of p and m must be 1:

 

(slope of p) * (-1/2) = -1

               

Slope of p = 2

So we must choose the equation that has a slope of 2. If we rewrite the equations in point-slope form (y = mx + b), we see that the equation 2x  y = 3 could be written as y = 2x – 3. This means that the slope of the line 2x – y =3 would be 2, so it could be the equation of line p. The answer is 2x – y = 3.

Example Question #1 : How To Find The Equation Of A Perpendicular Line

What is the equation for the line that is perpendicular to  through point ?

Possible Answers:

Correct answer:

Explanation:

Perpendicular slopes are opposite reciprocals.

The given slope is found by converting the equation to the slope-intercept form.

 

The slope of the given line is and the perpendicular slope is  .

We can use the given point and the new slope to find the perpendicular equation. Plug in the slope and the given coordinates to solve for the y-intercept.

Using this y-intercept in slope-intercept form, we get out final equation: .

Example Question #1 : How To Find The Equation Of A Perpendicular Line

Which line below is perpendicular to ?

Possible Answers:

Correct answer:

Explanation:

The definition of a perpendicular line is one that has a negative, reciprocal slope to another.

For this particular problem, we must first manipulate our initial equation into a more easily recognizable and useful form: slope-intercept form or .

According to our  formula, our slope for the original line is . We are looking for an answer that has a perpendicular slope, or an opposite reciprocal. The opposite reciprocal of  is . Flip the original and multiply it by

Our answer will have a slope of . Search the answer choices for  in the  position of the equation.

is our answer. 

(As an aside, the negative reciprocal of 4 is . Place the whole number over one and then flip/negate. This does not apply to the above problem, but should be understood to tackle certain permutations of this problem type where the original slope is an integer.)

Example Question #1 : How To Find The Equation Of A Perpendicular Line

If a line has an equation of 2y=3x+3, what is the slope of a line that is perpendicular to the line?

Possible Answers:

3

-\frac{2}{3}

\frac{3}{2}

-\frac{3}{2}

-2

Correct answer:

-\frac{2}{3}

Explanation:

Putting the first equation in slope-intercept form yields y=\frac{3}{2}x+\frac{3}{2}.

A perpendicular line has a slope that is the negative inverse. In this case, -\frac{2}{3}.

Example Question #3 : How To Find The Equation Of A Perpendicular Line

Which of the following is possibly a line perpendicular to ?

Possible Answers:

Correct answer:

Explanation:

To start, begin by dividing everything by , this will get your equation into the format .  This gives you:

Now, recall that the slope of a perpendicular line is the opposite and reciprocal slope to its mutually perpendicular line.  Thus, if our slope is , then the perpendicular line's slope must be .  Thus, we need to look at our answers to determine which equation has a slope of .  Among the options given, the only one that matches this is .  If you solve this for , you will get:

Example Question #1 : How To Find The Equation Of A Perpendicular Line

Which of the following is the equation of a line perpendicular to the line given by:

?

Possible Answers:

Correct answer:

Explanation:

For two lines to be perpendicular their slopes must have a product of .
 and so we see the correct answer is given by 

Example Question #3 : How To Find The Equation Of A Perpendicular Line

What is the equation of a line perpendicular to the line defined by the equaiton:

Possible Answers:

Correct answer:

Explanation:

Perpendicular lines have slopes whose product is .

Looking at our equations we can see that it is in slope-intercept form where the m value represents the slope of the line,

.

In our case we see that

 therefore, .

Since 

 we see the only possible answer is 

.

Example Question #11 : How To Find The Equation Of A Perpendicular Line

Line  follows the equation .

State the equation, in slope-intercept form, of line , which is perpendicular to line  and intersects it at point .

Possible Answers:

Correct answer:

Explanation:

Since line  follows the equation , we can surmise its slope is . Thus, it follows that any line perpendicular to  will have a slope of .

Since we also know at least one point on line , we can use point slope form to find an initial equation for our line.

----> .

Next, we can simplify to reach point-slope form.

---->.

Thus, line  in slope-intercept form is .

Example Question #21 : How To Find The Equation Of A Perpendicular Line

Which of the following equations represents a line that goes through the point  and is perpendicular to the line  ?

Possible Answers:

Correct answer:

Explanation:

In order to solve this problem, we need first to transform the equation from standard form to slope-intercept form:

Transform the original equation to find its slope.

 

First, subtract  from both sides of the equation.

Simplify and rearrange.

 

Next, divide both sides of the equation by 6.

The slope of our first line is equal to . Perpendicular lines have slopes that are opposite reciprocals of each other; therefore, if the slope of one is x, then the slope of the other is equal to the following: 

Let's calculate the opposite reciprocal of our slope:

The slope of our line is equal to 2. We now have the following partial equation:

We are missing the y-intercept, . Substitute the x- and y-values in the given point  to solve for the missing y-intercept. 

 

Add 4 to both sides of the equation.

Substitute this value into our partial equation to construct the equation of our line:

Example Question #1 : How To Find The Slope Of A Perpendicular Line

What is the slope of any line perpendicular to 2y = 4x +3 ?

 

Possible Answers:

– 4

2

– ½

½

Correct answer:

– ½

Explanation:

First, we must solve the equation for y to determine the slope:  y = 2x + 3/2

By looking at the coefficient in front of x, we know that the slope of this line has a value of 2. To fine the slope of any line perpendicular to this one, we take the negative reciprocal of it:

slope = m , perpendicular slope = – 1/m

slope = 2 , perpendicular slope = – 1/2

 

 

 

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