Solve r^2+12800r-4505600000=0

$$r ^ { 2 } + 12800 r - 4505600000 = 0$$

Solve for r

$$r=6400\sqrt{111}-6400\approx 61028.184018258$$

$$r=-6400\sqrt{111}-6400\approx -73828.184018258$$

Steps Using the Quadratic Formula

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.

$$r^{2}+12800r-4505600000=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute 1 for a, 12800 for b, and -4505600000 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$r=\frac{-12800±\sqrt{12800^{2}-4\left(-4505600000\right)}}{2}$$

Square 12800.

$$r=\frac{-12800±\sqrt{163840000-4\left(-4505600000\right)}}{2}$$

Multiply -4 times -4505600000.

$$r=\frac{-12800±\sqrt{163840000+18022400000}}{2}$$

Add 163840000 to 18022400000.

$$r=\frac{-12800±\sqrt{18186240000}}{2}$$

Take the square root of 18186240000.

$$r=\frac{-12800±12800\sqrt{111}}{2}$$

Now solve the equation r=\frac{-12800±12800\sqrt{111}}{2} when ± is plus. Add -12800 to 12800\sqrt{111}.

$$r=\frac{12800\sqrt{111}-12800}{2}$$

Divide -12800+12800\sqrt{111} by 2.

$$r=6400\sqrt{111}-6400$$

Now solve the equation r=\frac{-12800±12800\sqrt{111}}{2} when ± is minus. Subtract 12800\sqrt{111} from -12800.

$$r=\frac{-12800\sqrt{111}-12800}{2}$$

Divide -12800-12800\sqrt{111} by 2.

$$r=-6400\sqrt{111}-6400$$

The equation is now solved.

$$r=6400\sqrt{111}-6400 r=-6400\sqrt{111}-6400$$

Steps for Completing the Square

Quadratic equations such as this one can be solved by completing the square. In order to complete the square, the equation must first be in the form x^{2}+bx=c.

$$r^{2}+12800r-4505600000=0$$

Add 4505600000 to both sides of the equation.

$$r^{2}+12800r-4505600000-\left(-4505600000\right)=-\left(-4505600000\right)$$

Subtracting -4505600000 from itself leaves 0.

$$r^{2}+12800r=-\left(-4505600000\right)$$

Subtract -4505600000 from 0.

$$r^{2}+12800r=4505600000$$

Divide 12800, the coefficient of the x term, by 2 to get 6400. Then add the square of 6400 to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$r^{2}+12800r+6400^{2}=4505600000+6400^{2}$$

Square 6400.

$$r^{2}+12800r+40960000=4505600000+40960000$$

Add 4505600000 to 40960000.

$$r^{2}+12800r+40960000=4546560000$$

Factor r^{2}+12800r+40960000. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.

$$\left(r+6400\right)^{2}=4546560000$$

Take the square root of both sides of the equation.

$$\sqrt{\left(r+6400\right)^{2}}=\sqrt{4546560000}$$

Simplify.

$$r+6400=6400\sqrt{111} r+6400=-6400\sqrt{111}$$

Subtract 6400 from both sides of the equation.

$$r=6400\sqrt{111}-6400 r=-6400\sqrt{111}-6400$$

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.

$$x ^ 2 +12800x -1 = 0$$

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

$$r + s = -12800 rs = -1$$

Two numbers r and s sum up to -12800 exactly when the average of the two numbers is \frac{1}{2}*-12800 = -6400. 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>

$$r = -6400 - u s = -6400 + u$$

To solve for unknown quantity u, substitute these in the product equation rs = -1

$$(-6400 - u) (-6400 + u) = -1$$

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

$$40960000 - u^2 = -1$$

Simplify the expression by subtracting 40960000 on both sides

$$-u^2 = -1-40960000 = -40960001$$

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

$$u^2 = 40960001 u = \pm\sqrt{40960001} = \pm \sqrt{40960001}$$

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

$$r =-6400 - \sqrt{40960001} = -12800.000 s = -6400 + \sqrt{40960001} = 0.000$$