Factor the expression by grouping. First, the expression needs to be rewritten as $-x^{2}+ax+bx+8$. To find $a$ and $b$, set up a system to be solved.
$$a+b=2$$ $$ab=-8=-8$$
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 $-8$.
$$-1,8$$ $$-2,4$$
Calculate the sum for each pair.
$$-1+8=7$$ $$-2+4=2$$
The solution is the pair that gives sum $2$.
$$a=4$$ $$b=-2$$
Rewrite $-x^{2}+2x+8$ as $\left(-x^{2}+4x\right)+\left(-2x+8\right)$.
$$\left(-x^{2}+4x\right)+\left(-2x+8\right)$$
Factor out $-x$ in the first and $-2$ in the second group.
$$-x\left(x-4\right)-2\left(x-4\right)$$
Factor out common term $x-4$ by using distributive property.
$$\left(x-4\right)\left(-x-2\right)$$
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 $ax^{2}+bx+c=0$.
$$-x^{2}+2x+8=0$$
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.
Simplify all the expressions of the form $p-\left(-q\right)$ to $p+q$.
$$-x^{2}+2x+8=-\left(x+2\right)\left(x-4\right)$$
Steps Using Direct Factoring Method
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 $x^2+Bx+C=0$.
$$x ^ 2 -2x -8 = 0$$
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 $rs = C$
$$r + s = 2 $$ $$ rs = -8$$
Two numbers $r$ and $s$ sum up to $2$ exactly when the average of the two numbers is $\frac{1}{2}*2 = 1$. 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 $u$.
$$r = 1 - u$$ $$s = 1 + u$$
To solve for unknown quantity $u$, substitute these in the product equation $rs = -8$
$$(1 - u) (1 + u) = -8$$
Simplify by expanding $(a -b) (a + b) = a^2 – b^2$
$$1 - u^2 = -8$$
Simplify the expression by subtracting $1$ on both sides
$$-u^2 = -8-1 = -9$$
Simplify the expression by multiplying $-1$ on both sides and take the square root to obtain the value of unknown variable $u$
$$u^2 = 9$$ $$u = \pm\sqrt{9} = \pm 3 $$
The factors $r$ and $s$ are the solutions to the quadratic equation. Substitute the value of $u$ to compute the $r$ and $s$.
