Solve x^2+20x+99

$$x ^ { 2 } + 20 x + 99$$

Factor

$$\left(x+9\right)\left(x+11\right)$$

Solution Steps

Factor the expression by grouping. First, the expression needs to be rewritten as x^{2}+ax+bx+99. To find a and b, set up a system to be solved.

$$a+b=20 ab=1\times 99=99$$

Since ab is positive, a and b have the same sign. Since a+b is positive, a and b are both positive. List all such integer pairs that give product 99.

$$1,99 3,33 9,11$$

Calculate the sum for each pair.

$$1+99=100 3+33=36 9+11=20$$

The solution is the pair that gives sum 20.

$$a=9 b=11$$

Rewrite x^{2}+20x+99 as \left(x^{2}+9x\right)+\left(11x+99\right).

$$\left(x^{2}+9x\right)+\left(11x+99\right)$$

Factor out x in the first and 11 in the second group.

$$x\left(x+9\right)+11\left(x+9\right)$$

Factor out common term x+9 by using distributive property.

$$\left(x+9\right)\left(x+11\right)$$

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.

$$x^{2}+20x+99=0$$

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.

$$x=\frac{-20±\sqrt{20^{2}-4\times 99}}{2}$$

Square 20.

$$x=\frac{-20±\sqrt{400-4\times 99}}{2}$$

Multiply -4 times 99.

$$x=\frac{-20±\sqrt{400-396}}{2}$$

Add 400 to -396.

$$x=\frac{-20±\sqrt{4}}{2}$$

Take the square root of 4.

$$x=\frac{-20±2}{2}$$

Now solve the equation x=\frac{-20±2}{2} when ± is plus. Add -20 to 2.

$$x=-\frac{18}{2}$$

Divide -18 by 2.

$$x=-9$$

Now solve the equation x=\frac{-20±2}{2} when ± is minus. Subtract 2 from -20.

$$x=-\frac{22}{2}$$

Divide -22 by 2.

$$x=-11$$

Factor the original expression using ax^{2}+bx+c=a\left(x-x_{1}\right)\left(x-x_{2}\right). Substitute -9 for x_{1} and -11 for x_{2}.

$$x^{2}+20x+99=\left(x-\left(-9\right)\right)\left(x-\left(-11\right)\right)$$

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

$$x^{2}+20x+99=\left(x+9\right)\left(x+11\right)$$

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.

$$x ^ 2 +20x +99 = 0$$

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

$$r + s = -20 rs = 99$$

Two numbers r and s sum up to -20 exactly when the average of the two numbers is \frac{1}{2}*-20 = -10. 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>

$$r = -10 - u s = -10 + u$$

To solve for unknown quantity u, substitute these in the product equation rs = 99

$$(-10 - u) (-10 + u) = 99$$

Simplify by expanding (a -b) (a + b) = a^2 – b^2

$$100 - u^2 = 99$$

Simplify the expression by subtracting 100 on both sides

$$-u^2 = 99-100 = -1$$

Simplify the expression by multiplying -1 on both sides and take the square root to obtain the value of unknown variable u

$$u^2 = 1 u = \pm\sqrt{1} = \pm 1$$

The factors r and s are the solutions to the quadratic equation. Substitute the value of u to compute the r and s.

$$r =-10 - 1 = -11 s = -10 + 1 = -9$$

Evaluate

$$\left(x+9\right)\left(x+11\right)$$