Question

The value 4 is a lower bound for the zeros of the function shown below.

f(x) = 4x^3 – 12x^2 – x + 15

A) True
B) False​

Answer 1

False

Explanation:

f(x) = 4x³ - 12x² - x + 15

Set output to 0.

Factor the function.

0 = (x + 1)(2x - 3)(2x - 5)

Set factors equal to 0.

x + 1 = 0

x = -1

2x - 3 = 0

2x = 3

x = 3/2

2x - 5 = 0

2x = 5

x = 5/2

4 is not a lower bound for the zeros of the function.

Answer 2

False roots are x = -1 or x = 5/2 or x = 3/2

Explanation:

Solve for x:

4 x^3 - 12 x^2 - x + 15 = 0

The left hand side factors into a product with three terms:

(x + 1) (2 x - 5) (2 x - 3) = 0

Split into three equations:

x + 1 = 0 or 2 x - 5 = 0 or 2 x - 3 = 0

Subtract 1 from both sides:

x = -1 or 2 x - 5 = 0 or 2 x - 3 = 0

Add 5 to both sides:

x = -1 or 2 x = 5 or 2 x - 3 = 0

Divide both sides by 2:

x = -1 or x = 5/2 or 2 x - 3 = 0

Add 3 to both sides:

x = -1 or x = 5/2 or 2 x = 3

Divide both sides by 2:

Answer: x = -1 or x = 5/2 or x = 3/2