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

Question

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

Solution Steps

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 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