Question

How do you factor the expressions \[{{x}^{2}}+7x-18\]?

Answer 1

Hint: Use the middle term split method to factorize \[{{x}^{2}}+7x-18\]. Split 7x into two terms in such a way that their sum is 7x and the product is \[-18{{x}^{2}}\]. For this process, find the prime factors of 18 and combine them in such a way so that we can get our conditions satisfied. Finally, take common terms together and write \[{{x}^{2}}+7x-18\] as a product of two terms given as \[\left( x-a \right)\left( x-b \right)\]. Here, ‘a’ and ‘b’ are called zeroes of the polynomial.

Complete step by step answer:
Here, we have been asked to factorize the quadratic polynomial: \[{{x}^{2}}+7x-18\].
Let us use the middle term split method for the factorization. It says that we have to split the middle term which is 7x into two terms such that their sum is 7x and the product is \[-18{{x}^{2}}\]. To do this, first we need to find all the prime factors of 18. So, let us find it.
We know that 18 can be written as: - \[18=2\times 3\times 3\] as the product of its primes. Now, we have to group these factors in such a way that our conditions of the middle term split method is satisfied. So, we have,
(i) \[9x+\left( -2x \right)=7x\]
(ii) \[9x\times \left( -2x \right)=-18{{x}^{2}}\]
Hence, both the conditions of the middle terms split method are satisfied. So, the quadratic polynomial can be written as: -
\[\begin{align}
  & \Rightarrow {{x}^{2}}+7x-18={{x}^{2}}+9x-2x-18 \\
 & \Rightarrow {{x}^{2}}+7x-18=x\left( x+9 \right)-2\left( x+9 \right) \\
\end{align}\]
Taking \[\left( x+9 \right)\] common in the R.H.S., we get,
\[\Rightarrow {{x}^{2}}+7x-18=\left( x+9 \right)\left( x-2 \right)\]
Hence, \[\left( x+9 \right)\left( x-2 \right)\] is the factored form of the given quadratic polynomial.

Note:
 One may note that we can use another method for the factorization. The Discriminant method can also be applied to solve the question. What we will do is we will find the solution to the quadratic equation using the discriminant method. The values of x obtained will be assumed as x = a and x = b. Finally, we will consider the product \[\left( x-a \right)\left( x-b \right)\] to get the factored form.