Question

Find all x-intercepts of the function. Express your answer as a list of x-values.

f(x) = x3 – 4x2 – 16x + 64

Answer 1

Explanation:

one thing immediately catches the eye : the factors of the terms in the function follow the powers of 4 :

4⁰, 4¹, 4², 4³

and they are going against the powers of x :

x³ has the factor 4⁰

x² has the factor 4¹

x has the factor 4²

x⁰ has the factor 4³

with that we see that one 0 solution (an x- intercept) is x = 4.

f(4) = 4³ - 4×4² - 4²×4 + 4³ = 4³ - 4³ - 4³ + 4³ = 0

so, the functional expression is

(x-4)(x² + ...)

the other 2 zero solutions are covered by the quadratic term.

to get the quadratic term we divide f(x) by (x-4).

x³ - 4x² - 16x + 64 ÷ x - 4 = x² - 16

- x³ - 4x²

-----------------------------

0 0 - 16x + 64

- - 16x + 64

----------------------‐--------

0 0

so,

f(x) = (x - 4)(x² - 16)

and as we know

(a² - b²) = (a + b)(a - b)

we get

f(x) = (x - 4)(x + 4)(x - 4) = (x - 4)²(x + 4)

so, x = 4 counts as 2 zero solutions, it is a point, where the curve makes a turn and only touches the x-axis, but does not fully intercept it (in the sense to continue on the other side of the x-axis).

and x = -4 is the third zero solution.

so, the x-intercepts are at x = -4, +4

Answer 2

all x intercepts [-4, 0] [4, 0]

Explanation:

x-intercept, i.e. y=0

f(x) = x^3 - 4x^2 - 16x + 64 = 0

(x^3 + 4^3) - 4x^2 - 16x = 0 [FYI, x^3 + y^3 = (x + y)(x^2 - xy + y^2)]

(x + 4)(x^2 - 4x + 16) - 4x^2 - 16x = 0

(x + 4)(x^2 - 4x + 16) - 4x(x + 4) = 0

(x + 4)(x^2 - 4x + 16 - 4x) = 0

(x + 4)(x^2 - 8x + 16) = 0

(x + 4)(x - 4)^2 = 0

x = -4 or 4

all x intercepts [-4, 0] [4, 0]