Question

Which of the following polynomial equations has 1, 4, -5 as three of its roots?

Answer 1

Answer: x³-21x+20=0

Step-by-step explanation:

To find which polynomial has 1, 4, -5 as the roots, all we have to do is equal each root to 0 and multiply all factors together.

1 is (x-1)=0

4 is (x-4)=0

-5 is (x+5)=0

Now, we just multiply them together.

(x-1)(x-4)(x+5)=0                       [FOIL]

(x²-4x-x+4)(x+5)=0                    [combine like terms]

(x²-5x+4)(x+5)=0                      [FOIL]

x³+5x²-5x²-25x+4x+20=0       [combine like terms]

x³-21x+20=0

Now we know x³-21x+20=0 is the polynomial with those roots.