4x3 + 12x2 − x − 3

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kingattaskus12's profile pic

kingattaskus12 | (Level 3) Adjunct Educator

Posted on

Find every combination of `+-p/q`  These are the possible roots of the polynomial function.

`+-1, +- 1/2,+- 1/4,+-3,+-3/2,+-3/4`  


Since `1/2`  is a known root, divide the polynomial by `(x - 1/2)`  to find the quotient polynomial. This polynomial can then be used to find the remaining roots.

`(4x^3+ 12x^2 - x - 3)/(x - 1/2)`


Next, find the roots of the remaining polynomial. The order of the polynomial has been reduced by `1` .

`4x^2 + 14x + 6`


The polynomial can be written as a set of linear factors.

`(x - 1/2) (x + 3) (x + 1/2 )`


These are the roots (i.e. zeros) of the polynomial `4x^3 + 12x^2 - x - 3`

`x = 1/2 , - 3 , -1/2`

A graph is attached below

` `

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valentin68 | College Teacher | (Level 3) Associate Educator

Posted on

To factorize the above expression we group two by two the terms

`4x^3 +12x^2 -x -3 =4x^2(x+3) -(x+3) =(4x^2-1)(x+3) =`

`=(2x+1)(2x-1)(x+3)`

Therefore the roots of the equation

`4x^3 +12x^2 -x -3 =0`

are the roots of the equivalent equation

`(2x+1)(2x-1)(x+3) =0`

`x_1 =-1/2` , `x_2 =1/2` , `x_3 =-3`

The graph of the function `f(x) =4x^3 +12x^2 -x -3` is attached below.

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