Perpendicular Lines - PSAT Math

Perpendicular lines have slopes that are the negative reciprocals of each other.

The line which is perpendicular has a slope which is the negative inverse of the slope of the original line.

Two lines are perpendicular to each other if their slopes are negative reciprocals. For example, if one line has a slope of 2, the line perpendicular to it has a slope of –1/2. One easy way to eliminate answer choices is to check if the slopes have the same sign, i.e. both positive or both negative. If so, they cannot be perpendicular. Several of the lines in the answer choices are of the form y = mx + b, where m is the slope and b is the y-intercept. We are only worried about the slope for the purposes of this question.

y = 3_x_ + 5 and y = 5_x_ + 3 both have positive slopes (m = 3 and m = 5, respectively), so they aren't perpendicular.

y = 3_x_/5 – 3 and y = 5_x_/3 + 3 both have positive slopes, so again they aren't perpendicular.

y = x – 1/2 and y = –x + 1/2 have slopes of m = 1 and m = –1, respectively. One is positive and one is negative, so that is a good sign. Let's take the negative reciprocal of 1. 1 /–1 = –1. So these two slopes are in fact negative reciprocals, and these two lines are perpendicular to each other. Even though we have found the correct answer, let's go through the other two choices to be sure.

The line between the points (1,3) and (3,5), and y = 4_x_ + 7: We need to find the slope of the first line. slope = rise / run = (_y_2 – _y_1) / (_x_2 – x_1) = (5 – 3) / (3 – 1) = 1. The slope of y = 4_x + 7 is also positive (m = 4), so the lines are not perpendicular.

The line between the points (7,4) and (4,7), and the line between the points (3,9) and (4,8): the first slope = (7 – 4) / (4 – 7) = –1 and the second slope = (8 – 9) / (4 – 3) = –1. They have the same slope, making them parallel, not perpendicular.

We calculate the slopes of the lines using the slope formula.

The slope of line is

The slope of line is

Parallel lines and identical lines must have the same slope, so these can be eliminated as choices. The slopes of perpendicular lines must have product . The slopes have product

so they are not perpendicular.

The correct response is that the lines are distinct but neither parallel nor perpendicular.

We calculate the slopes of the lines using the slope formula.

The slope of line is

The slope of line is

The slopes are not the same, so the lines are neither parallel nor identical. We multiply their slopes to test for perpendicularity:

The product of the slopes is , making the lines perpendicular.

We find the slope of each line by putting each equation in slope-intercept form and examining the coefficient of .

is already in slope-intercept form; its slope is .

To get in slope-intercept form we solve for :

The slope of this line is also .

The slopes are equal; however, the -intercepts are different - the -intercept of the first line is and that of the second line is . Therefore, the lines are parallel as opposed to being the same line.

To determine if lines are perpendicular, look at the slope. Perpendicular lines have slopes that are negative reciprocals. The slope of is . The negative reciprocal of is . Therefore, the answer is .

First, put the equation of the line given into slope-intercept form by solving for y. You get y = -2_x_ +5, so the slope is –2. Perpendicular lines have opposite-reciprocal slopes, so the slope of the line we want to find is 1/2. Plugging in the point given into the equation y = 1/2_x_ + b and solving for b, we get b = 6. Thus, the equation of the line is y = ½_x_ + 6. Rearranged, it is –x/2 + y = 6.

Using the slope intercept formula, we can see the slope of line p is ¼. Since line k is perpendicular to line p it must have a slope that is the negative reciprocal. (-4/1) If we set up the formula y=mx+b, using the given point and a slope of (-4), we can solve for our b or y-intercept. In this case it would be 17.

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First one needs to use one of the two equations to substitute one of the unknowns.

From the second equation we can derive that y = x – 3.

Then we substitute what we got into the first equation which gives us: x + x – 3 = 15.

Next we solve for x, so 2_x_ = 18 and x = 9.

x – y = 3, so y = 6.

These two lines will intersect at the point (9,6).

We start by add x to the other side of the equation to get the y by itself, giving us 8y =17 + x. We then divide both sides by 8, giving us y= 17/8 + 1/8x. Since we are looking for the equation of a perpendicular line, we know the slope (the coefficient in front of x) will be the opposite reciprocal of the slope of our line, giving us y= -8x + 5 as the answer.

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:

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.

First, you need to obtain the equation of the first line, A. Its slope is given by:

(y2 - y1) / (x2 - x1) = (1 - 0) / (3 - 0) = 1/3 = slope of A.

Remember that the slope of a perpendicular line to a given line is -1 times the inverse of its slope. Thus the slope of B:

(-1) x 1 / (1/3) = -3

Thus with y = mx + b, m = -3. Now the line must include (3,1). Thus:

with y = -3x + b:

1 = -3(3) + b;

1 = -9 + b; add 9 to both sides:

10 = b

The given equation is in standard form, so it must be converted to slope-intercept form: y = mx + b to discover the slope is –2/3. To be perpendicular the new slope must be 3/2 (opposite reciprocal of the old slope). Using the new slope and the given point we can substitute these values back into the slope-intercept form to find the new intercept, –5. In slope-intercept form the new equation is y = 3/2x – 5. The correct answer is this equation converted to standard form.

We are asked to find the equation of the line that is the perpendicular bisector of AB. If we find a point that the line passes through as well as its slope, we can determine its equation. In order for the line to bisect AB, it must pass through the midpoint of AB. Thus, one point on the line is the midpoint of the AB. We can use the midpoint formula to determine the midpoint of AB with endpoints (5, –2) and (–3, 10).

The x-coordinate of the midpoint is located at (5 + –3)/2 = 1.

The y-coordinate of the midpoint is located at (–2 + 10)/2 = 4.

Thus, the midpoint of AB is (1, 4).

So, we know that the line passes through (1,4). Now, we can use the fact that the line is perpendicular to AB to find its slope. The product of the slopes of two line segments that are perpendicular is equal to –1. In other words, if we multiply the slope of the line by the slope of AB, we will get –1.

We can use the slope formula to find the slope of AB.

slope of AB = (10 – (–2))/(–3 – 5) = 12/–8 = –3/2.

Since the slope of the line multiplied by –3/2 must equal –1, we can write the following:

(slope of the line)(–3/2) = –1

If we multiply both sides by –2/3, we will find the slope of the line.

The slope of the line = (–1)(–2/3) = 2/3.

Thus, the line passes through the ponit (1, 4) and has a slope of 2/3.

We will now use point-slope form to determine the line's equation. Let's let m represent the slope and (x1, y1) represent a ponit on the line.

y – y = m(x – x1)

y – 4 = (2/3)(x – 1)

Multiply both sides by 3 to get rid of the fraction.

3(y – 4) = 2(x – 1)

Distribute both sides.

3y – 12 = 2x – 2

Subtract 3y from both sides.

–12= 2x – 3y – 2

Add 2 to both sides.

–10 = 2x – 3y.

The equation of the line is 2x – 3y = –10.

The answer is 2x – 3y = –10.

Remember, perpendicular lines have opposite-reciprocal slopes; therefore, let's first find the slope of our line. That is found by the equation: rise/run or y2 – y1/x2 – x1

Substituting in our values: (15 – 8)/(4 – 2) = 7/2

The perpendicular slope is therefore –2/7.

Since ANY perpendicular line will intersect with this line at some point. We merely need to choose the answer that has a line with slope –2/7. Following the slope intercept form (y = mx + b), we know that the coefficient of x will give us this; therefore our answer is: y = (–2/7)x + 4

Convert the equation to slope intercept form to get y = –1/3_x_ + 2. The old slope is –1/3 and the new slope is 3. Perpendicular slopes must be opposite reciprocals of each other: _m_1 * _m_2 = –1

With the new slope, use the slope intercept form and the point to calculate the intercept: y = mx + b or 5 = 3(1) + b, so b = 2

So y = 3_x_ + 2

The equation of line p is given in the form y = mx + b, where m is the slope and b is the y-intercept. Because the equation is y = –x + 4, the slope is m = –1.

If two lines perpendicular, then the product of their slopes is equal to –1. Thus, if we call n the slope of a line perpendicular to line p, then the following equation is true:

m(n) = –1

Because the slope of line p is –1, we can write (–1)n = –1. If we divide both sides by –1, then n = 1. In short, the slope of a line perpendicular to line p must equal 1. We are looking for the equation of a line whose slope equals 1.

Let's examine the answer choices. The equation y = –x – 4 is in the form y = mx + b (which is called point-slope form), so its slope is –1, not 1. Thus, we can eliminate this choice.

Next, let's look at the line x + y = 4. This line is in the form Ax + By = C, where A, B, and C are constants. When a line is in this form, its slope is equal to –A/B. Therefore, the slope of this line is equal to –1/1 = –1, which isn't 1. So we can eliminate x + y = 4. Simiarly, we can eliminate the line x + y = –4.

The line y = –4 is a horizontal line, so its slope is 0, which isn't 1.

The answer is the line y = x + 4, because it is the only line with a slope of 1.

The answer is y = x + 4.