SOLUTION: x+1=9x^3 + 9x^2 solve the polynomial equation. Been working on this one for awhile and i cant seem to get this one. 4 answers it needs.

Algebra ->  Equations -> SOLUTION: x+1=9x^3 + 9x^2 solve the polynomial equation. Been working on this one for awhile and i cant seem to get this one. 4 answers it needs.       Log On


   



Question 223550: x+1=9x^3 + 9x^2 solve the polynomial equation.
Been working on this one for awhile and i cant seem to get this one. 4 answers it needs.

Answer by Edwin McCravy(20055) About Me  (Show Source):
You can put this solution on YOUR website!
x+1=9x^3 + 9x^2 solve the polynomial equation.
Been working on this one for awhile and i cant seem to get this one. 4 answers it needs.

No, it only needs 3 "answers", "solutions" or "roots", whichever
label you wish to give them.  That's because the degree (largest
exponent of x) is 3.

x%2B1=9x%5E3+%2B+9x%5E2

Get zero on the right:

-9x%5E3+-+9x%5E2%2Bx%2B1=0

Multiply through by -1 so leading coefficient will be
positive:


9x%5E3+%2B+9x%5E2-x-1=0

Possible rational zeros are

±1, ±1%2F3, and ±1%2F9

By DesCartes' rule of signs, there is one positive
answer (solution or root), and 2 or 0 negative ones.

Let's try x=1, that is, divide by x-1 to see if
1 is a solution:

1 | 9   9   -1   -1
  |     9  -18  -19
   ----------------
    9 -18  -19  -20

Nope, the remainder is -20, not zero, so x-1 is not a
divisor of 9x%5E3+%2B+9x%5E2-x-1, and so x=1 is not
a solution.

Let's try x=-1, that is, divide by x+1 to see if
-1 is a solution:

-1 | 9   9  -1 -1
   |    -9   0  1
   --------------
     9   0  -1  0

Aha! The remainder is zero, so x+1 is a divisor of
of 9x%5E3+%2B+9x%5E2-x-1, and therefore x=-1 is a
solution.

Anytime we divide by synthetic division and get a zero 
remainder, we have factored the polynomial.  In this
case we have factored 9x%5E3+%2B+9x%5E2-x-1 and now
we have

%28x%2B1%29%289x%5E2%2B0x-1%29=0

or eliminating the 0x term:

%28x%2B1%29%289x%5E2-1%29=0

Now we can factor the expression in the second parentheses
as the difference of two perfect squares and get:

%28x%2B1%29%283x-1%29%283x%2B1%29=0

Setting each factor = 0,

x+1=0 gives x=-1

3x-1=0 gives x=1%2F3

3x+1=0 gives x=-1%2F3

Edwin