Factor the expression by grouping. First, the expression needs to be rewritten as $n^{2}+an+bn-24$. To find $a$ and $b$, set up a system to be solved.
$$a+b=10$$ $$ab=1\left(-24\right)=-24$$
Since $ab$ is negative, $a$ and $b$ have the opposite signs. Since $a+b$ is positive, the positive number has greater absolute value than the negative. List all such integer pairs that give product $-24$.
$$-1,24$$ $$-2,12$$ $$-3,8$$ $$-4,6$$
Calculate the sum for each pair.
$$-1+24=23$$ $$-2+12=10$$ $$-3+8=5$$ $$-4+6=2$$
The solution is the pair that gives sum $10$.
$$a=-2$$ $$b=12$$
Rewrite $n^{2}+10n-24$ as $\left(n^{2}-2n\right)+\left(12n-24\right)$.
$$\left(n^{2}-2n\right)+\left(12n-24\right)$$
Factor out $n$ in the first and $12$ in the second group.
$$n\left(n-2\right)+12\left(n-2\right)$$
Factor out common term $n-2$ by using distributive property.
$$\left(n-2\right)\left(n+12\right)$$
Steps Using the Quadratic Formula
Quadratic polynomial can be factored using the transformation $ax^{2}+bx+c=a\left(x-x_{1}\right)\left(x-x_{2}\right)$, where $x_{1}$ and $x_{2}$ are the solutions of the quadratic equation $ax^{2}+bx+c=0$.
$$n^{2}+10n-24=0$$
All equations of the form $ax^{2}+bx+c=0$ can be solved using the quadratic formula: $\frac{-b±\sqrt{b^{2}-4ac}}{2a}$. The quadratic formula gives two solutions, one when $±$ is addition and one when it is subtraction.
Quadratic equations such as this one can be solved by a new direct factoring method that does not require guess work. To use the direct factoring method, the equation must be in the form $x^2+Bx+C=0$.
$$x ^ 2 +10x -24 = 0$$
Let $r$ and $s$ be the factors for the quadratic equation such that $x^2+Bx+C=(x−r)(x−s)$ where sum of factors $(r+s)=−B$ and the product of factors $rs = C$
$$r + s = -10 $$ $$ rs = -24$$
Two numbers $r$ and $s$ sum up to $-10$ exactly when the average of the two numbers is $\frac{1}{2}*-10 = -5$. You can also see that the midpoint of $r$ and $s$ corresponds to the axis of symmetry of the parabola represented by the quadratic equation $y=x^2+Bx+C$. The values of $r$ and $s$ are equidistant from the center by an unknown quantity $u$. Express $r$ and $s$ with respect to variable $u$.
$$r = -5 - u$$ $$s = -5 + u$$
To solve for unknown quantity $u$, substitute these in the product equation $rs = -24$
$$(-5 - u) (-5 + u) = -24$$
Simplify by expanding $(a -b) (a + b) = a^2 – b^2$
$$25 - u^2 = -24$$
Simplify the expression by subtracting $25$ on both sides
$$-u^2 = -24-25 = -49$$
Simplify the expression by multiplying $-1$ on both sides and take the square root to obtain the value of unknown variable $u$
$$u^2 = 49$$ $$u = \pm\sqrt{49} = \pm 7 $$
The factors $r$ and $s$ are the solutions to the quadratic equation. Substitute the value of $u$ to compute the $r$ and $s$.
