Solve n^2-10n-24

$$n ^ { 2 } - 10 n - 24$$

Factor

$$\left(n-12\right)\left(n+2\right)$$

Solution Steps

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 negative, the negative number has greater absolute value than the positive. 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.

$$a=-12 b=2$$

Rewrite n^{2}-10n-24 as \left(n^{2}-12n\right)+\left(2n-24\right).

$$\left(n^{2}-12n\right)+\left(2n-24\right)$$

Factor out n in the first and 2 in the second group.

$$n\left(n-12\right)+2\left(n-12\right)$$

Factor out common term n-12 by using distributive property.

$$\left(n-12\right)\left(n+2\right)$$

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.

$$n^{2}-10n-24=0$$

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.

$$n=\frac{-\left(-10\right)±\sqrt{\left(-10\right)^{2}-4\left(-24\right)}}{2}$$

Square -10.

$$n=\frac{-\left(-10\right)±\sqrt{100-4\left(-24\right)}}{2}$$

Multiply -4 times -24.

$$n=\frac{-\left(-10\right)±\sqrt{100+96}}{2}$$

Add 100 to 96.

$$n=\frac{-\left(-10\right)±\sqrt{196}}{2}$$

Take the square root of 196.

$$n=\frac{-\left(-10\right)±14}{2}$$

The opposite of -10 is 10.

$$n=\frac{10±14}{2}$$

Now solve the equation n=\frac{10±14}{2} when ± is plus. Add 10 to 14.

$$n=\frac{24}{2}$$

Divide 24 by 2.

$$n=12$$

Now solve the equation n=\frac{10±14}{2} when ± is minus. Subtract 14 from 10.

$$n=-\frac{4}{2}$$

Divide -4 by 2.

$$n=-2$$

Factor the original expression using ax^{2}+bx+c=a\left(x-x_{1}\right)\left(x-x_{2}\right). Substitute 12 for x_{1} and -2 for x_{2}.

$$n^{2}-10n-24=\left(n-12\right)\left(n-\left(-2\right)\right)$$

Simplify all the expressions of the form p-\left(-q\right) to p+q.

$$n^{2}-10n-24=\left(n-12\right)\left(n+2\right)$$

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.

$$x ^ 2 -10x -24 = 0$$

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. <div style='padding: 8px'><img src='https://opalmath.azureedge.net/customsolver/quadraticgraph.png' style='width: 100%;max-width: 700px' /></div>

$$r = 5 - u s = 5 + u$$

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.

$$r =5 - 7 = -2 s = 5 + 7 = 12$$

Evaluate

$$\left(n-12\right)\left(n+2\right)$$