To solve the equation, factor $x^{2}-4x-21$ using formula $x^{2}+\left(a+b\right)x+ab=\left(x+a\right)\left(x+b\right)$. To find $a$ and $b$, set up a system to be solved.
$$a+b=-4$$ $$ab=-21$$
Since $ab$ is negative, $a$ and $b$ have the opposite signs. Since $a+b$ is negative, the negative number has greater absolute value than the positive. List all such integer pairs that give product $-21$.
$$1,-21$$ $$3,-7$$
Calculate the sum for each pair.
$$1-21=-20$$ $$3-7=-4$$
The solution is the pair that gives sum $-4$.
$$a=-7$$ $$b=3$$
Rewrite factored expression $\left(x+a\right)\left(x+b\right)$ using the obtained values.
$$\left(x-7\right)\left(x+3\right)$$
To find equation solutions, solve $x-7=0$ and $x+3=0$.
$$x=7$$ $$x=-3$$
Steps Using Factoring By Grouping
To solve the equation, factor the left hand side by grouping. First, left hand side needs to be rewritten as $x^{2}+ax+bx-21$. To find $a$ and $b$, set up a system to be solved.
$$a+b=-4$$ $$ab=1\left(-21\right)=-21$$
Since $ab$ is negative, $a$ and $b$ have the opposite signs. Since $a+b$ is negative, the negative number has greater absolute value than the positive. List all such integer pairs that give product $-21$.
$$1,-21$$ $$3,-7$$
Calculate the sum for each pair.
$$1-21=-20$$ $$3-7=-4$$
The solution is the pair that gives sum $-4$.
$$a=-7$$ $$b=3$$
Rewrite $x^{2}-4x-21$ as $\left(x^{2}-7x\right)+\left(3x-21\right)$.
$$\left(x^{2}-7x\right)+\left(3x-21\right)$$
Factor out $x$ in the first and $3$ in the second group.
$$x\left(x-7\right)+3\left(x-7\right)$$
Factor out common term $x-7$ by using distributive property.
$$\left(x-7\right)\left(x+3\right)$$
To find equation solutions, solve $x-7=0$ and $x+3=0$.
$$x=7$$ $$x=-3$$
Steps Using the Quadratic Formula
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.
$$x^{2}-4x-21=0$$
This equation is in standard form: $ax^{2}+bx+c=0$. Substitute $1$ for $a$, $-4$ for $b$, and $-21$ for $c$ in the quadratic formula, $\frac{-b±\sqrt{b^{2}-4ac}}{2a}$.
Now solve the equation $x=\frac{4±10}{2}$ when $±$ is plus. Add $4$ to $10$.
$$x=\frac{14}{2}$$
Divide $14$ by $2$.
$$x=7$$
Now solve the equation $x=\frac{4±10}{2}$ when $±$ is minus. Subtract $10$ from $4$.
$$x=-\frac{6}{2}$$
Divide $-6$ by $2$.
$$x=-3$$
The equation is now solved.
$$x=7$$ $$x=-3$$
Steps for Completing the Square
Quadratic equations such as this one can be solved by completing the square. In order to complete the square, the equation must first be in the form $x^{2}+bx=c$.
Divide $-4$, the coefficient of the $x$ term, by $2$ to get $-2$. Then add the square of $-2$ to both sides of the equation. This step makes the left hand side of the equation a perfect square.
Factor $x^{2}-4x+4$. In general, when $x^{2}+bx+c$ is a perfect square, it can always be factored as $\left(x+\frac{b}{2}\right)^{2}$.
$$\left(x-2\right)^{2}=25$$
Take the square root of both sides of the equation.
$$\sqrt{\left(x-2\right)^{2}}=\sqrt{25}$$
Simplify.
$$x-2=5$$ $$x-2=-5$$
Add $2$ to both sides of the equation.
$$x=7$$ $$x=-3$$
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 $x^2+Bx+C=0$.
$$x ^ 2 -4x -21 = 0$$
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 $rs = C$
$$r + s = 4 $$ $$ rs = -21$$
Two numbers $r$ and $s$ sum up to $4$ exactly when the average of the two numbers is $\frac{1}{2}*4 = 2$. 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 $u$.
$$r = 2 - u$$ $$s = 2 + u$$
To solve for unknown quantity $u$, substitute these in the product equation $rs = -21$
$$(2 - u) (2 + u) = -21$$
Simplify by expanding $(a -b) (a + b) = a^2 – b^2$
$$4 - u^2 = -21$$
Simplify the expression by subtracting $4$ on both sides
$$-u^2 = -21-4 = -25$$
Simplify the expression by multiplying $-1$ on both sides and take the square root to obtain the value of unknown variable $u$
$$u^2 = 25$$ $$u = \pm\sqrt{25} = \pm 5 $$
The factors $r$ and $s$ are the solutions to the quadratic equation. Substitute the value of $u$ to compute the $r$ and $s$.
