Find the Linear Function with Slope ±2 and Domain x > 3

Linear Functions with Domain Restrictions

The following is a function that is positive in domain:

3<x 3 < x

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

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:00 Find the function equations
00:03 The function is positive when it's above the X-axis
00:09 Let's draw the function
00:17 and negative when the function is below the X-axis
00:23 For the function to be positive according to the given data
00:27 The slope must be positive
00:43 We'll use the linear equation to find the function
00:50 We'll substitute appropriate values to find B
01:01 Let's isolate B
01:06 This is the unknown B, now let's substitute and find the linear function
01:12 And this is the solution to the question

Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

The following is a function that is positive in domain:

3<x 3 < x

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

2

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 .

3

Final Answer

y=2x6 y=2x-6

Key Points to Remember

Essential concepts to master this topic
  • Slope Rule: Absolute value |m| = 2 means slope is +2 or -2
  • Domain Check: For x > 3, substitute x = 4: y = 2(4) - 6 = 2 > 0
  • Verification: Function y = 2x - 6 crosses x-axis at x = 3, positive for x > 3 ✓

Common Mistakes

Avoid these frequent errors
  • Choosing negative slope without checking domain positivity
    Don't pick y = -2x + 6 just because |slope| = 2! This function becomes negative when x > 3, violating the positive domain requirement. Always test both slope options (+2 and -2) to see which keeps the function positive in the given domain.

Practice Quiz

Test your knowledge with interactive questions

Look at the function shown in the figure.

When is the function positive?

xy-4-7

FAQ

Everything you need to know about this question

Why can't the slope be -2 if the absolute value is 2?

+

The slope could be -2, but we need to check the domain requirement. With y=2x+c y = -2x + c , the function decreases as x increases. For the function to be positive when x>3 x > 3 , we'd need it to stay above the x-axis, which doesn't work with a negative slope in this range.

How do I know which y-intercept to use?

+

Look at the answer choices! The problem gives you specific options to test. With slope = 2, check each y-intercept by substituting x=4 x = 4 (slightly above 3) to see if y>0 y > 0 .

What does 'positive in domain x > 3' actually mean?

+

It means for every x-value greater than 3, the y-value must be positive. So when x=3.1,4,5,100 x = 3.1, 4, 5, 100 , etc., we need y>0 y > 0 .

Why is x = 3 important if the domain is x > 3?

+

Even though x=3 x = 3 isn't in the domain, it's the boundary point. For y=2x6 y = 2x - 6 , when x=3 x = 3 , y=0 y = 0 . This means the function crosses the x-axis at x = 3 and becomes positive immediately after.

Can I just guess from the answer choices?

+

While you could test each choice, it's better to understand the logic! Start with the slope requirement, then check which option stays positive for x>3 x > 3 . This builds your problem-solving skills for harder questions.

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