Linear Functions Analysis: Which Functions are Positive When x > 2?

Linear Functions with Domain Restrictions

Choose the functions that fit the following description:

The function is positive in the domain 2<x 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

❤️ Continue Your Math Journey!

We have hundreds of course questions with personalized recommendations + Account 100% premium

Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:00 Which functions are positive in the given domain?
00:03 The function is positive when Y is greater than 0
00:11 The slope is greater than 0
00:18 Let's find the intersection point with the X-axis, set Y = 0
00:23 This is the intersection point with the X-axis
00:30 Since the slope is positive, the function is positive from the intersection point
00:34 Therefore it must also be positive in the given domain
00:40 Let's examine the next function, using the same method
00:48 Let's find the intersection point with the X-axis
00:53 This is the intersection point with the X-axis
00:55 Since the slope is positive, the function is positive from the intersection point
00:58 Let's examine the next function, using the same method
01:07 Let's find the intersection point with the X-axis
01:10 This is the intersection point with the X-axis
01:13 Since the slope is positive, the function is positive from the intersection point
01:18 Let's examine the next function, using the same method
01:27 Let's find the intersection point with the X-axis
01:36 This is the intersection point with the X-axis
01:40 Since the slope is positive, the function is positive from the intersection point
01:43 Let's examine the next function, using the same method
01:56 Let's find the intersection point with the X-axis
02:05 Let's find the intersection point with the X-axis
02:08 Since the slope is positive, the function is positive from the intersection point
02:11 The intersection point is greater than the given value
02:15 Therefore the function is not positive in the given domain

Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

Choose the functions that fit the following description:

The function is positive in the domain 2<x 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

2

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 .

3

Final Answer

a, b, d, e

Key Points to Remember

Essential concepts to master this topic
  • Rule: A function is positive when its output values are greater than zero
  • Technique: Test boundary value: y = 2x - 4 at x = 2 gives y = 0
  • Check: Verify with x = 3: positive slope functions increase, negative slope decrease ✓

Common Mistakes

Avoid these frequent errors
  • Only checking the function value at x = 2
    Don't just evaluate at x = 2 and assume that tells you about x > 2! A function might equal zero at x = 2 but still be positive for x > 2. Always consider both the value at the boundary AND the slope direction.

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 does y = 2x - 4 count if it equals zero at x = 2?

+

Great question! The domain is x > 2, not x ≥ 2. Since the slope is positive (+2), the function becomes positive immediately after x = 2, even though it equals zero exactly at x = 2.

How do I quickly tell if a linear function will be positive for large x values?

+

Look at the slope (the coefficient of x)! If the slope is positive, the function will eventually become positive as x increases. If negative, it will become negative.

What about y = 5x - 14? It's negative at x = 2 but still counts?

+

Yes! Even though y=5(2)14=4 y = 5(2) - 14 = -4 is negative at x = 2, the positive slope means it crosses the x-axis and becomes positive soon after. It's positive for all x > 2.8.

Why doesn't y = -2x + 4 work if it equals zero at x = 2?

+

The key is the negative slope (-2). Even though it equals zero at x = 2, it becomes negative for x > 2 because the line is decreasing. Try x = 3: y = -2(3) + 4 = -2.

Is there a faster way than testing multiple x values?

+

Yes! Find where each function equals zero (set y = 0), then use the slope to determine behavior. Positive slope: positive after the zero. Negative slope: negative after the zero.

What makes y = 2 always work?

+

It's a constant function - it always equals 2 regardless of the x value. Since 2 > 0, this function is positive everywhere, including when x > 2.

🌟 Unlock Your Math Potential

Get unlimited access to all 18 Linear Functions questions, detailed video solutions, and personalized progress tracking.

📹

Unlimited Video Solutions

Step-by-step explanations for every problem

📊

Progress Analytics

Track your mastery across all topics

🚫

Ad-Free Learning

Focus on math without distractions

No credit card required • Cancel anytime

More Questions

Click on any question to see the complete solution with step-by-step explanations