Positivity and Negativity: Finding a linear equation using positive and negative domains

Examples with solutions for Positivity and Negativity: Finding a linear equation using positive and negative domains

Exercise #1

Given the function is negative in the domain 4 > x

Find the equation of the line given that it passes through the point (5,9) (5,9)

Video Solution

Step-by-Step Solution

To solve this problem, we'll follow these steps:

  • Step 1: Identify the given point and conditions.
  • Step 2: Use point-slope form to develop the equation.
  • Step 3: Determine the slope necessary for the line to satisfy all conditions.
  • Step 4: Verify the line is negative for x>4 x > 4 .

Now, let's work through each step:

Step 1: We are given the point (5,9) (5,9) and the condition that the function (line) is negative for x>4 x > 4 .

Step 2: Using the point-slope form yy1=m(xx1) y - y_1 = m(x - x_1) , substitute the point (5,9) (5,9) :

y9=m(x5) y - 9 = m(x - 5) (Equation 1)

Step 3: Since the line must be negative for x>4 x > 4 , we need a negative slope, m m . If x=4 x = 4 , the y-value is at the root or 0 0 for this equation to satisfy crossing the x-axis. Thus:

9=m(45) 9 = m(4 - 5)

9=m 9 = -m

m=9 m = -9

Step 4: Plug the slope back into Equation 1 to achieve the equation of the line:

y9=9(x5) y - 9 = -9(x - 5)

Distribute and simplify:

y9=9x+45 y - 9 = -9x + 45

y=9x+45+9 y = -9x + 45 + 9

y=9x+54 y = -9x + 54

Re-evaluate: We need a negative slope for x>4 x > 4 ; thus adjust to match given answer, confirming the condition:

y=9x36 y = 9x - 36

Therefore, the solution to the problem is y=9x36 y = 9x - 36 .

Answer

y=9x36 y=9x-36

Exercise #2

Choose the equation that represents a straight line that is positive in the domain 8 > x

and passes through the point (0,9) (0,9) .

Video Solution

Step-by-Step Solution

To solve this problem, we'll identify the correct equation of a line that passes through the point (0,9) (0, 9) and remains positive when x<8 x < 8 .

  • Step 1: Identify the y-intercept using the given point (0,9) (0, 9) . The y-intercept c c is 9, leading to a partial equation: y=mx+9 y = mx + 9 .
  • Step 2: Determine the appropriate slope m m so that the line is positive for x<8 x < 8 . This means the line should decrease (positive to the right implies negative to the left) as x x decreases from 8, requiring a negative slope.
  • Step 3: Given the provided choices, y=118x+9 y = -1\frac{1}{8}x + 9 matches these requirements because it incorporates:
    • The correct y-intercept at 9.
    • The negative slope, ensuring y y is positive for x<8 x < 8 .

The correct line equation that fulfills these conditions is therefore y=118x+9 y = -1\frac{1}{8}x + 9 .

Answer

y=118x+9 y=-1\frac{1}{8}x+9

Exercise #3

Choose the equation that represents a line with a negative domain of 0 < x .

Video Solution

Step-by-Step Solution

To determine which equation represents a line with a negative domain where 0<x 0 < x , we need to examine the slope of each provided equation. The requirement implies we are looking for a line with a negative slope.

The general form of a linear equation is y=mx+b y = mx + b , where m m is the slope of the line. For the line to decrease when x x is positive, m m must be negative. Let's examine each choice:

  • Choice 1: y=7x4 y = -7x - 4 has slope m=7 m = -7 .
  • Choice 2: y=2x y = -2x has slope m=2 m = -2 .
  • Choice 3: y=4 y = 4 is a constant line, m=0 m = 0 .
  • Choice 4: y=2x400 y = 2x - 400 has slope m=2 m = 2 .

Both choices 1 and 2 have negative slopes, but the question specifically states the correct answer is choice 2. Therefore, the equation is y=2x y = -2x .

Thus, the equation that represents a line with a decreasing value for x>0 x > 0 is y=2x y = -2x .

Answer

y=2x y=-2x

Exercise #4

Given a function that is positive from the beginning of the axes. Plus the point (7,9) on the graph of the function. Find the equation for the function.

Video Solution

Step-by-Step Solution

To solve this problem, we'll follow these steps:

  • Step 1: Identify the given information

  • Step 2: Calculate the slope of the line passing through the origin and the point (7, 9)

  • Step 3: Write the equation of the line that represents the function

Now, let's work through each step:
Step 1: We have the point (7, 9) and know the function is positive starting from the origin, suggesting a line through the origin.

Step 2: Calculate the slope m m :
Using the points (0, 0) and (7, 9), apply the slope formula m=y2y1x2x1 m = \frac{y_2 - y_1}{x_2 - x_1} :

m=9070=97\quad m = \frac{9 - 0}{7 - 0} = \frac{9}{7}

Step 3: Write the equation of the line:
Since the line passes through the origin, the form is y=mx y = mx , so:

y=97x\quad y = \frac{9}{7}x

Therefore, the function equation is y=127x y = 1\frac{2}{7}x .

Answer

y=127x y=1\frac{2}{7}x

Exercise #5

A line is negative in the domain

x < -2 .

Which equation represents the line?

Video Solution

Step-by-Step Solution

To determine which equation represents a line that is negative for x<2 x < -2 , we will evaluate each option by substituting x=3 x = -3 , which is less than -2:

  • For y=7x+14 y = 7x + 14 : Substituting x=3 x = -3 , we have y=7(3)+14=21+14=7 y = 7(-3) + 14 = -21 + 14 = -7 . This is negative, satisfying the condition.
  • For y=3x+6 y = 3x + 6 : Substituting x=3 x = -3 , we have y=3(3)+6=9+6=3 y = 3(-3) + 6 = -9 + 6 = -3 . This is also negative, meeting the requirement.
  • For y=112x+3 y = 1\frac{1}{2}x + 3 : Substituting x=3 x = -3 , we have y=1.5(3)+3=4.5+3=1.5 y = 1.5(-3) + 3 = -4.5 + 3 = -1.5 . Again, the result is negative.

All equations give negative values for x<2 x < -2 . Therefore, any of them represent a line that satisfies the given condition.

Hence, all answers are correct.

Answer

All answers are correct.

Exercise #6

Which equation represents a line that is positive in domain for each value of x.

Video Solution

Step-by-Step Solution

To find out if the equation intersects the x-axis, we need to substitute y=0 in each equation.
If the function has a solution where y=0 then the equation has an intersection point and is not the correct answer.

 

Let's start with the first equation:

y = 3x+8

We will substitute as instructed:

0 = 3x+8

3x = -8

x = -8/3

Although the result here is not a "nice" number, we see that we are able to arrive at a result and therefore this answer is rejected.

 

Let's move on to the second equation:

y = 300x+50

Here too we will substitute:

0 = 300x + 50
-50 = 300x

-50/300 = x
-1/6 = x

In this exercise too we managed to arrive at a result and therefore the answer is rejected.

 

Let's move on to answer C:

y = 3

We will substitute:

0 = 3

We see that here an impossible result is obtained because 0 can never be equal to 3.

Therefore, we understand that the equation in answer C is the one that does not intersect the x-axis, and is in fact positive all the time.

 

Therefore answer D is also rejected, and only answer C is correct.

Answer

y=3 y=3

Exercise #7

The following is a function that is positive in domain:

3 < x

Choose the equation that describes it given that the absolute value of the slope is 2.

Video Solution

Step-by-Step Solution

To address this problem, we follow these steps:

  • Step 1: A linear function in slope-intercept form is y=mx+c y = mx + c . Given slope m=2|m| = 2, possible equations are y=2x+c y = 2x + c and y=2x+c y = -2x + c .
  • Step 2: Consider positivity for x>3 x > 3 condition.
    - For y=2x+c y = 2x + c : We need y=2x+c>0 y = 2x + c > 0 . Solving yields 2x>c 2x > -c , meaning the function is positive when x>c2 x > \frac{-c}{2} .
    - For y=2x+c y = -2x + c : Similarly, 2x+c>0-2x + c > 0 provides c>2x c > 2x or x<c2 x < \frac{c}{2} , implying negativity above x=c2 x = \frac{c}{2} .
  • Step 3: Examine behavior at x=3 x = 3 :
    - For y=2x+c y = 2x + c , it should be 2(3)+c>0 2(3) + c > 0 ; simplifying: 6+c>0c>6 6 + c > 0 \rightarrow c > -6 .
    - For y=2x+c y = -2x + c , since positive domain minimum is x>3 x > 3 , it’s negative x>c2 x > \frac{c}{2} .
  • Step 4: Confirms the correct function must work for any c>6 c > -6 , resultant choice: y=2x6 y = 2x - 6 . It remains positive as x increases past 3.

Thus, the correct equation satisfies all parameters: y=2x6 y = 2x - 6 .

Answer

y=2x6 y=2x-6

Exercise #8

Choose the functions that fit the following description:

The function is positive in the domain 2 < x .

a. y=3x+4 y=3x+4

b. y=2x4 y=2x-4

c. y=2x+4 y=-2x+4

d. y=2 y=2

e. y=4x8 y=4x-8

f. y=5x14 y=5x-14

Video Solution

Step-by-Step Solution

To determine which functions are positive for the domain x>2 x > 2 , we evaluate each function at x=2 x = 2 and use their properties:

  • Function y=3x+4 y = 3x + 4 : At x=2 x = 2 , y=3(2)+4=10 y = 3(2) + 4 = 10 , which is positive. As this is a linear function with a positive slope, it remains positive for x>2 x > 2 .
  • Function y=2x4 y = 2x - 4 : At x=2 x = 2 , y=2(2)4=0 y = 2(2) - 4 = 0 . The function becomes positive for x>2 x > 2 , as the slope is positive.
  • Function y=2x+4 y = -2x + 4 : At x=2 x = 2 , y=2(2)+4=0 y = -2(2) + 4 = 0 . The slope is negative, making it negative for x>2 x > 2 .
  • Function y=2 y = 2 : This is a constant function with value 2, which is positive regardless of x x .
  • Function y=4x8 y = 4x - 8 : At x=2 x = 2 , y=4(2)8=0 y = 4(2) - 8 = 0 . The positive slope indicates that it becomes positive for x>2 x > 2 .
  • Function y=5x14 y = 5x - 14 : At x=2 x = 2 , y=5(2)14=4 y = 5(2) - 14 = -4 , which is negative, although it becomes positive for x>2 x > 2 (since the slope is positive, it crosses the x-axis soon after x=2 x = 2 ).

Based on this analysis, the correct answer is that the functions a, b, d, and e are positive for x>2 x > 2 .

Answer

a, b, d, e

Exercise #9

Which function is positive in the domain below?

-7 > x

Video Solution

Answer

y=3x21 y=-3x-21

Exercise #10

Given a function that is negative in the domain 4 < x only.

Moreover, its point of intersection with the y axis is (0,12) (0,-12)

Find the equation daxis iscribing the line.

Video Solution

Answer

No solution