Solve 5x^2+45x+100=

$$5 x ^ { 2 } + 45 x + 100 =$$

Factor

$$5\left(x+4\right)\left(x+5\right)$$

Solution Steps

Factor out 5.

$$5\left(x^{2}+9x+20\right)$$

Consider x^{2}+9x+20. Factor the expression by grouping. First, the expression needs to be rewritten as x^{2}+ax+bx+20. To find a and b, set up a system to be solved.

$$a+b=9 ab=1\times 20=20$$

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 20.

$$1,20 2,10 4,5$$

Calculate the sum for each pair.

$$1+20=21 2+10=12 4+5=9$$

The solution is the pair that gives sum 9.

$$a=4 b=5$$

Rewrite x^{2}+9x+20 as \left(x^{2}+4x\right)+\left(5x+20\right).

$$\left(x^{2}+4x\right)+\left(5x+20\right)$$

Factor out x in the first and 5 in the second group.

$$x\left(x+4\right)+5\left(x+4\right)$$

Factor out common term x+4 by using distributive property.

$$\left(x+4\right)\left(x+5\right)$$

Rewrite the complete factored expression.

$$5\left(x+4\right)\left(x+5\right)$$

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.

$$5x^{2}+45x+100=0$$

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.

$$x=\frac{-45±\sqrt{45^{2}-4\times 5\times 100}}{2\times 5}$$

Square 45.

$$x=\frac{-45±\sqrt{2025-4\times 5\times 100}}{2\times 5}$$

Multiply -4 times 5.

$$x=\frac{-45±\sqrt{2025-20\times 100}}{2\times 5}$$

Multiply -20 times 100.

$$x=\frac{-45±\sqrt{2025-2000}}{2\times 5}$$

Add 2025 to -2000.

$$x=\frac{-45±\sqrt{25}}{2\times 5}$$

Take the square root of 25.

$$x=\frac{-45±5}{2\times 5}$$

Multiply 2 times 5.

$$x=\frac{-45±5}{10}$$

Now solve the equation x=\frac{-45±5}{10} when ± is plus. Add -45 to 5.

$$x=-\frac{40}{10}$$

Divide -40 by 10.

$$x=-4$$

Now solve the equation x=\frac{-45±5}{10} when ± is minus. Subtract 5 from -45.

$$x=-\frac{50}{10}$$

Divide -50 by 10.

$$x=-5$$

Factor the original expression using ax^{2}+bx+c=a\left(x-x_{1}\right)\left(x-x_{2}\right). Substitute -4 for x_{1} and -5 for x_{2}.

$$5x^{2}+45x+100=5\left(x-\left(-4\right)\right)\left(x-\left(-5\right)\right)$$

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

$$5x^{2}+45x+100=5\left(x+4\right)\left(x+5\right)$$

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.This is achieved by dividing both sides of the equation by 5

$$x ^ 2 +9x +20 = 0$$

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

$$r + s = -9 rs = 20$$

Two numbers r and s sum up to -9 exactly when the average of the two numbers is \frac{1}{2}*-9 = -\frac{9}{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>

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

To solve for unknown quantity u, substitute these in the product equation rs = 20

$$(-\frac{9}{2} - u) (-\frac{9}{2} + u) = 20$$

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

$$\frac{81}{4} - u^2 = 20$$

Simplify the expression by subtracting \frac{81}{4} on both sides

$$-u^2 = 20-\frac{81}{4} = -\frac{1}{4}$$

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

$$u^2 = \frac{1}{4} u = \pm\sqrt{\frac{1}{4}} = \pm \frac{1}{2}$$

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

$$r =-\frac{9}{2} - \frac{1}{2} = -5 s = -\frac{9}{2} + \frac{1}{2} = -4$$

Evaluate

$$5\left(x+4\right)\left(x+5\right)$$