Solve c^2-11c+18

$$c ^ { 2 } - 11 c + 18$$

Factor

$$\left(c-9\right)\left(c-2\right)$$

Solution Steps

Factor the expression by grouping. First, the expression needs to be rewritten as c^{2}+ac+bc+18. To find a and b, set up a system to be solved.

$$a+b=-11 ab=1\times 18=18$$

Since ab is positive, a and b have the same sign. Since a+b is negative, a and b are both negative. List all such integer pairs that give product 18.

$$-1,-18 -2,-9 -3,-6$$

Calculate the sum for each pair.

$$-1-18=-19 -2-9=-11 -3-6=-9$$

The solution is the pair that gives sum -11.

$$a=-9 b=-2$$

Rewrite c^{2}-11c+18 as \left(c^{2}-9c\right)+\left(-2c+18\right).

$$\left(c^{2}-9c\right)+\left(-2c+18\right)$$

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

$$c\left(c-9\right)-2\left(c-9\right)$$

Factor out common term c-9 by using distributive property.

$$\left(c-9\right)\left(c-2\right)$$

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.

$$c^{2}-11c+18=0$$

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.

$$c=\frac{-\left(-11\right)±\sqrt{\left(-11\right)^{2}-4\times 18}}{2}$$

Square -11.

$$c=\frac{-\left(-11\right)±\sqrt{121-4\times 18}}{2}$$

Multiply -4 times 18.

$$c=\frac{-\left(-11\right)±\sqrt{121-72}}{2}$$

Add 121 to -72.

$$c=\frac{-\left(-11\right)±\sqrt{49}}{2}$$

Take the square root of 49.

$$c=\frac{-\left(-11\right)±7}{2}$$

The opposite of -11 is 11.

$$c=\frac{11±7}{2}$$

Now solve the equation c=\frac{11±7}{2} when ± is plus. Add 11 to 7.

$$c=\frac{18}{2}$$

Divide 18 by 2.

$$c=9$$

Now solve the equation c=\frac{11±7}{2} when ± is minus. Subtract 7 from 11.

$$c=\frac{4}{2}$$

Divide 4 by 2.

$$c=2$$

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 2 for x_{2}.

$$c^{2}-11c+18=\left(c-9\right)\left(c-2\right)$$

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.

$$x ^ 2 -11x +18 = 0$$

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

$$r + s = 11 rs = 18$$

Two numbers r and s sum up to 11 exactly when the average of the two numbers is \frac{1}{2}*11 = \frac{11}{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>

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

To solve for unknown quantity u, substitute these in the product equation rs = 18

$$(\frac{11}{2} - u) (\frac{11}{2} + u) = 18$$

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

$$\frac{121}{4} - u^2 = 18$$

Simplify the expression by subtracting \frac{121}{4} on both sides

$$-u^2 = 18-\frac{121}{4} = -\frac{49}{4}$$

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

$$u^2 = \frac{49}{4} u = \pm\sqrt{\frac{49}{4}} = \pm \frac{7}{2}$$

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

$$r =\frac{11}{2} - \frac{7}{2} = 2 s = \frac{11}{2} + \frac{7}{2} = 9$$

Evaluate

$$\left(c-9\right)\left(c-2\right)$$