Question

Which of the following represents the zeros of f(x) = x3 − 11x2 + 38x − 40?

Answer 1

The answer is 5, 4, 2

Among all choices we have 5, so
x = 5
x - 5 = 0
Let's divide the expression by (x - 5) using the long division:
                          x³ - 11x² + 38x - 40
(x - 5)  * x² =      x³ - 5x²                   Subtract
____________________________
                              -6x² + 38x - 40
   (x - 5) * (-6x) =    -6x² + 30x          Subtract
____________________________
                                           8x - 40
                    (x - 5) * 8 =     8x - 40   Sutract
____________________________
                                              0

Thus: x³ - 11x² + 38x - 40 = (x - 5)(x² - 6x + 8)

Now, let's simplify x² - 6x + 8.

x² - 6x + 8 = x² - 2x - 4x + 8 =
                  = x² - 2*x - (4*x - 4*2) = 
                  = x(x - 2) - 4(x - 2) =
                  = (x - 4)(x - 2)

Hence:
x³ - 11x² + 38x - 40 = (x - 5)(x - 4)(x - 2)
To calculate zero:
x³ - 11x² + 38x - 40 = 0
(x - 5)(x - 4)(x - 2) = 0
x - 5 = 0            or             x - 4 = 0              or      x - 2 = 0
x = 5                 or              x = 4                   or      x = 2