Solve D^2+200D+400000=0

$$D ^ { 2 } + 200 D + 400000 = 0$$

Solve for D

$$D=-100+100\sqrt{39}i\approx -100+624.49979984i$$

$$D=-100\sqrt{39}i-100\approx -100-624.49979984i$$

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.

$$D^{2}+200D+400000=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute 1 for a, 200 for b, and 400000 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$D=\frac{-200±\sqrt{200^{2}-4\times 400000}}{2}$$

Square 200.

$$D=\frac{-200±\sqrt{40000-4\times 400000}}{2}$$

Multiply -4 times 400000.

$$D=\frac{-200±\sqrt{40000-1600000}}{2}$$

Add 40000 to -1600000.

$$D=\frac{-200±\sqrt{-1560000}}{2}$$

Take the square root of -1560000.

$$D=\frac{-200±200\sqrt{39}i}{2}$$

Now solve the equation D=\frac{-200±200\sqrt{39}i}{2} when ± is plus. Add -200 to 200i\sqrt{39}.

$$D=\frac{-200+200\sqrt{39}i}{2}$$

Divide -200+200i\sqrt{39} by 2.

$$D=-100+100\sqrt{39}i$$

Now solve the equation D=\frac{-200±200\sqrt{39}i}{2} when ± is minus. Subtract 200i\sqrt{39} from -200.

$$D=\frac{-200\sqrt{39}i-200}{2}$$

Divide -200-200i\sqrt{39} by 2.

$$D=-100\sqrt{39}i-100$$

The equation is now solved.

$$D=-100+100\sqrt{39}i D=-100\sqrt{39}i-100$$

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.

$$D^{2}+200D+400000=0$$

Subtract 400000 from both sides of the equation.

$$D^{2}+200D+400000-400000=-400000$$

Subtracting 400000 from itself leaves 0.

$$D^{2}+200D=-400000$$

Divide 200, the coefficient of the x term, by 2 to get 100. Then add the square of 100 to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$D^{2}+200D+100^{2}=-400000+100^{2}$$

Square 100.

$$D^{2}+200D+10000=-400000+10000$$

Add -400000 to 10000.

$$D^{2}+200D+10000=-390000$$

Factor D^{2}+200D+10000. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.

$$\left(D+100\right)^{2}=-390000$$

Take the square root of both sides of the equation.

$$\sqrt{\left(D+100\right)^{2}}=\sqrt{-390000}$$

Simplify.

$$D+100=100\sqrt{39}i D+100=-100\sqrt{39}i$$

Subtract 100 from both sides of the equation.

$$D=-100+100\sqrt{39}i D=-100\sqrt{39}i-100$$

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.

$$x ^ 2 +200x +400000 = 0$$

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

$$r + s = -200 rs = 400000$$

Two numbers r and s sum up to -200 exactly when the average of the two numbers is \frac{1}{2}*-200 = -100. 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>

$$r = -100 - u s = -100 + u$$

To solve for unknown quantity u, substitute these in the product equation rs = 400000

$$(-100 - u) (-100 + u) = 400000$$

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

$$10000 - u^2 = 400000$$

Simplify the expression by subtracting 10000 on both sides

$$-u^2 = 400000-10000 = 390000$$

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

$$u^2 = -390000 u = \pm\sqrt{-390000} = \pm \sqrt{390000}i$$

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

$$r =-100 - \sqrt{390000}i s = -100 + \sqrt{390000}i$$