Solve -x^2+13x-22

$$- x ^ { 2 } + 13 x - 22$$

Factor

$$\left(2-x\right)\left(x-11\right)$$

Solution Steps

Factor the expression by grouping. First, the expression needs to be rewritten as -x^{2}+ax+bx-22. To find a and b, set up a system to be solved.

$$a+b=13 ab=-\left(-22\right)=22$$

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 22.

$$1,22 2,11$$

Calculate the sum for each pair.

$$1+22=23 2+11=13$$

The solution is the pair that gives sum 13.

$$a=11 b=2$$

Rewrite -x^{2}+13x-22 as \left(-x^{2}+11x\right)+\left(2x-22\right).

$$\left(-x^{2}+11x\right)+\left(2x-22\right)$$

Factor out -x in the first and 2 in the second group.

$$-x\left(x-11\right)+2\left(x-11\right)$$

Factor out common term x-11 by using distributive property.

$$\left(x-11\right)\left(-x+2\right)$$

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.

$$-x^{2}+13x-22=0$$

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.

$$x=\frac{-13±\sqrt{13^{2}-4\left(-1\right)\left(-22\right)}}{2\left(-1\right)}$$

Square 13.

$$x=\frac{-13±\sqrt{169-4\left(-1\right)\left(-22\right)}}{2\left(-1\right)}$$

Multiply -4 times -1.

$$x=\frac{-13±\sqrt{169+4\left(-22\right)}}{2\left(-1\right)}$$

Multiply 4 times -22.

$$x=\frac{-13±\sqrt{169-88}}{2\left(-1\right)}$$

Add 169 to -88.

$$x=\frac{-13±\sqrt{81}}{2\left(-1\right)}$$

Take the square root of 81.

$$x=\frac{-13±9}{2\left(-1\right)}$$

Multiply 2 times -1.

$$x=\frac{-13±9}{-2}$$

Now solve the equation x=\frac{-13±9}{-2} when ± is plus. Add -13 to 9.

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

Divide -4 by -2.

$$x=2$$

Now solve the equation x=\frac{-13±9}{-2} when ± is minus. Subtract 9 from -13.

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

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 2 for x_{1} and 11 for x_{2}.

$$-x^{2}+13x-22=-\left(x-2\right)\left(x-11\right)$$

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.

$$x ^ 2 -13x +22 = 0$$

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

$$r + s = 13 rs = 22$$

Two numbers r and s sum up to 13 exactly when the average of the two numbers is \frac{1}{2}*13 = \frac{13}{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. <div style='padding: 8px'><img src='https://opalmath.azureedge.net/customsolver/quadraticgraph.png' style='width: 100%;max-width: 700px' /></div>

$$r = \frac{13}{2} - u s = \frac{13}{2} + u$$

To solve for unknown quantity u, substitute these in the product equation rs = 22

$$(\frac{13}{2} - u) (\frac{13}{2} + u) = 22$$

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

$$\frac{169}{4} - u^2 = 22$$

Simplify the expression by subtracting \frac{169}{4} on both sides

$$-u^2 = 22-\frac{169}{4} = -\frac{81}{4}$$

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

$$u^2 = \frac{81}{4} u = \pm\sqrt{\frac{81}{4}} = \pm \frac{9}{2}$$

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

$$r =\frac{13}{2} - \frac{9}{2} = 2 s = \frac{13}{2} + \frac{9}{2} = 11$$

Evaluate

$$\left(2-x\right)\left(x-11\right)$$