If 2 and -2 are two zeros of f(x)=x^3-3x^2-4x+12, then the third zero is?

A:3 B:4 C:6 D:8 E:12

Question

If 2 and -2 are two zeros of f(x)=x^3-3x^2-4x+12, then the third zero is?

A:3   B:4   C:6   D:8    E:12

amtran_bus · Answer

Is there any way I could use the quadratic formula?

amtran_bus · Answer

@campbell_st @Jhannybean

campbell_st · Answer

well I'd factorise it can be done by grouping in pairs

$$f(x) = x^3 - 3x^2  - 4x + 12  $$
 
 $$f(x)     = x^2(x - 3) - 4(x - 3)$$

which simplifies to 

$$f(x) = (x^2 - 4)(x - 3)$$

hope this helps

amtran_bus · Answer

Can you use the quadratic formula at all? Don't you use it for zeros?  @campbell_st

campbell_st · Answer

you could if you had a quadratic factor... since this is a cubic... the quadratic formula only works after finding a linear factor.

anonymous · Answer

c

amtran_bus · Answer

Got you.

amtran_bus · Answer

@maxromeo , it is A

amtran_bus · Answer

How would you solve it step by step, after what you did campbell?

jhannybean · Answer

f(x)=(x-3)(x^2-4)=0    solve each part for x

(x-3)=0 and (x^2-4)=0
(x-3)=0
x=3

(x^2-4)=0 
x^2=4
x= +-2

amtran_bus · Answer

Thanks