Question

How do you factor ${{x}^{3}}+{{x}^{2}}-9x-9$ by grouping?

Answer 1

Hint: First we explain the process of grouping. We take the common divisors out to form one more common term. We then apply the identity of ${{a}^{2}}-{{b}^{2}}=\left( a+b \right)\left( a-b \right)$ to complete the factorisation.

Complete step-by-step solution:
Factorising a polynomial by grouping is to find the pairs which on taking their common divisor out, give the same remaining number.
In case of ${{x}^{3}}+{{x}^{2}}-9x-9$, the grouping will be done for ${{x}^{3}}+{{x}^{2}}$ and $-9x-9$.
We try to take the common numbers out.
For ${{x}^{3}}+{{x}^{2}}$, we take ${{x}^{2}}$ and get ${{x}^{2}}\left( x+1 \right)$.
For $-9x-9$, we take $-9$ and get $-9\left( x+1 \right)$.
The equation becomes ${{x}^{3}}+{{x}^{2}}-9x-9={{x}^{2}}\left( x+1 \right)-9\left( x+1 \right)$.
Both the terms have $\left( x+1 \right)$ in common. We take that term again and get
$\begin{align}
  & {{x}^{3}}+{{x}^{2}}-9x-9 \\
 & ={{x}^{2}}\left( x+1 \right)-9\left( x+1 \right) \\
 & =\left( x+1 \right)\left( {{x}^{2}}-9 \right) \\
\end{align}$
Although the grouping is completed, the factorisation is still remaining.
Now we find the factorisation of the equation ${{x}^{2}}-9$ using the identity of ${{a}^{2}}-{{b}^{2}}=\left( a+b \right)\left( a-b \right)$. We convert to the equation of ${{x}^{2}}-9={{x}^{2}}-{{3}^{2}}$.
Therefore, we get ${{x}^{2}}-{{3}^{2}}=\left( x+3 \right)\left( x-3 \right)$. The individual terms are in their primitive form. We can’t further break those polynomials.
The factorisation is ${{x}^{3}}+{{x}^{2}}-9x-9=\left( x+1 \right)\left( {{x}^{2}}-9 \right)=\left( x+1 \right)\left( x+3 \right)\left( x-3 \right)$.

Note: We have one more condition to check if the grouping is possible or not. If we order the individual elements of the polynomial according to their power of variables, then the multiple of end terms will be equal to the multiple of middle terms.
The given equation is in the form of ${{x}^{3}}+{{x}^{2}}-9x-9$. We take the multiple of two end terms and the multiple of two middle terms. Both multiplications give the result of $\left( -9 \right)\times \left( {{x}^{3}} \right)=\left( -9x \right)\times \left( {{x}^{2}} \right)=-9{{x}^{3}}$.