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October 1, 2014

October 1, 2014

Posted by **Anonymous** on Monday, March 25, 2013 at 7:53pm.

X^4+x^3=16-8x-6x^2. Answer: -2,1,+-√8

3x^3+4x^2+6=x. (-2,1/3, i√8/3)

Please help so I know how to figure these equations out.A big THANKS

- college algebra -
**Reiny**, Monday, March 25, 2013 at 8:50pmX^4+x^3=16-8x-6x^2

x^4 + x^3 + 6x^2 + 8x - 16 = 0

you must know the factor theorem to do these

look at the constant of 16

if there are factors of the type ( ?x + c) , c must be a factor of 16

so try ±1, ±2, ±4, ... that is, try factors of 16

e.g.

let x = 1 , we get

1 + 1 + 6 + 8 - 16 = 0 ahhhh, so x-1 is a factor, that was lucky

try some more:

x = 2, we get

16 + 8 + 24 + 16 - 16 ≠ 0

x = -2

16 - 8 + 24 - 16 - 16 = 0 , aahhh again, so x+2 is a factor

now do a long algebraic division by first x-1, and then x+2

both must divide evenly.

I got

x^4 + x^3 + 6x^2 + 8x - 16 = (x-1)(x+2)(x^2 + 8)

so x = 1 , -2 or

x^2 = -8 = 2√-2 = ±2√2 i , yours should have been ±√8 i

2nd:

3x^3 + 4x^2 - x + 6 = 0

try x = ±1, ±2, ±3

x=1 ---> 1 + 4 - 1 + 6 ≠0

x=-1 --> -3 + 4 + 1 + 6 ≠ 0

x = 2 --> 24 + 16 ...... ≠ 0

x = -2 --> -24 + 16 + 2 + 6 = 0 ..... finally, x+2 is a factor

division:

3x^3 + 4x^2 - x + 6 = (x+2)(3x^2 - 2x + 3)

so x = -2

or

x = (2 ±√-32)/6

= ( 2 ± 4√2 i)/6

= (1 ± 2√2 i)/3

I don't see how you got your answers.

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