Solve x^2+25x+66

$$x ^ { 2 } + 25 x + 66$$

Factor

$$\left(x+3\right)\left(x+22\right)$$

Solution Steps

Factor the expression by grouping. First, the expression needs to be rewritten as x^{2}+ax+bx+66. To find a and b, set up a system to be solved.

$$a+b=25 ab=1\times 66=66$$

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 66.

$$1,66 2,33 3,22 6,11$$

Calculate the sum for each pair.

$$1+66=67 2+33=35 3+22=25 6+11=17$$

The solution is the pair that gives sum 25.

$$a=3 b=22$$

Rewrite x^{2}+25x+66 as \left(x^{2}+3x\right)+\left(22x+66\right).

$$\left(x^{2}+3x\right)+\left(22x+66\right)$$

Factor out x in the first and 22 in the second group.

$$x\left(x+3\right)+22\left(x+3\right)$$

Factor out common term x+3 by using distributive property.

$$\left(x+3\right)\left(x+22\right)$$

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.

$$x^{2}+25x+66=0$$

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.

$$x=\frac{-25±\sqrt{25^{2}-4\times 66}}{2}$$

Square 25.

$$x=\frac{-25±\sqrt{625-4\times 66}}{2}$$

Multiply -4 times 66.

$$x=\frac{-25±\sqrt{625-264}}{2}$$

Add 625 to -264.

$$x=\frac{-25±\sqrt{361}}{2}$$

Take the square root of 361.

$$x=\frac{-25±19}{2}$$

Now solve the equation x=\frac{-25±19}{2} when ± is plus. Add -25 to 19.

$$x=-\frac{6}{2}$$

Divide -6 by 2.

$$x=-3$$

Now solve the equation x=\frac{-25±19}{2} when ± is minus. Subtract 19 from -25.

$$x=-\frac{44}{2}$$

Divide -44 by 2.

$$x=-22$$

Factor the original expression using ax^{2}+bx+c=a\left(x-x_{1}\right)\left(x-x_{2}\right). Substitute -3 for x_{1} and -22 for x_{2}.

$$x^{2}+25x+66=\left(x-\left(-3\right)\right)\left(x-\left(-22\right)\right)$$

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

$$x^{2}+25x+66=\left(x+3\right)\left(x+22\right)$$

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.

$$x ^ 2 +25x +66 = 0$$

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

$$r + s = -25 rs = 66$$

Two numbers r and s sum up to -25 exactly when the average of the two numbers is \frac{1}{2}*-25 = -\frac{25}{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>

$$r = -\frac{25}{2} - u s = -\frac{25}{2} + u$$

To solve for unknown quantity u, substitute these in the product equation rs = 66

$$(-\frac{25}{2} - u) (-\frac{25}{2} + u) = 66$$

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

$$\frac{625}{4} - u^2 = 66$$

Simplify the expression by subtracting \frac{625}{4} on both sides

$$-u^2 = 66-\frac{625}{4} = -\frac{361}{4}$$

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

$$u^2 = \frac{361}{4} u = \pm\sqrt{\frac{361}{4}} = \pm \frac{19}{2}$$

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

$$r =-\frac{25}{2} - \frac{19}{2} = -22 s = -\frac{25}{2} + \frac{19}{2} = -3$$

Evaluate

$$\left(x+3\right)\left(x+22\right)$$