Solve -x^2+5x=100

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

Solve for x (complex solution)

$$x=\frac{-5\sqrt{15}i+5}{2}\approx 2.5-9.682458366i$$

$$x=\frac{5+5\sqrt{15}i}{2}\approx 2.5+9.682458366i$$

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.

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

Subtract 100 from both sides of the equation.

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

Subtracting 100 from itself leaves 0.

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

This equation is in standard form: ax^{2}+bx+c=0. Substitute -1 for a, 5 for b, and -100 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-5±\sqrt{5^{2}-4\left(-1\right)\left(-100\right)}}{2\left(-1\right)}$$

Square 5.

$$x=\frac{-5±\sqrt{25-4\left(-1\right)\left(-100\right)}}{2\left(-1\right)}$$

Multiply -4 times -1.

$$x=\frac{-5±\sqrt{25+4\left(-100\right)}}{2\left(-1\right)}$$

Multiply 4 times -100.

$$x=\frac{-5±\sqrt{25-400}}{2\left(-1\right)}$$

Add 25 to -400.

$$x=\frac{-5±\sqrt{-375}}{2\left(-1\right)}$$

Take the square root of -375.

$$x=\frac{-5±5\sqrt{15}i}{2\left(-1\right)}$$

Multiply 2 times -1.

$$x=\frac{-5±5\sqrt{15}i}{-2}$$

Now solve the equation x=\frac{-5±5\sqrt{15}i}{-2} when ± is plus. Add -5 to 5i\sqrt{15}.

$$x=\frac{-5+5\sqrt{15}i}{-2}$$

Divide -5+5i\sqrt{15} by -2.

$$x=\frac{-5\sqrt{15}i+5}{2}$$

Now solve the equation x=\frac{-5±5\sqrt{15}i}{-2} when ± is minus. Subtract 5i\sqrt{15} from -5.

$$x=\frac{-5\sqrt{15}i-5}{-2}$$

Divide -5-5i\sqrt{15} by -2.

$$x=\frac{5+5\sqrt{15}i}{2}$$

The equation is now solved.

$$x=\frac{-5\sqrt{15}i+5}{2} x=\frac{5+5\sqrt{15}i}{2}$$

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.

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

Divide both sides by -1.

$$\frac{-x^{2}+5x}{-1}=\frac{100}{-1}$$

Dividing by -1 undoes the multiplication by -1.

$$x^{2}+\frac{5}{-1}x=\frac{100}{-1}$$

Divide 5 by -1.

$$x^{2}-5x=\frac{100}{-1}$$

Divide 100 by -1.

$$x^{2}-5x=-100$$

Divide -5, the coefficient of the x term, by 2 to get -\frac{5}{2}. Then add the square of -\frac{5}{2} to both sides of the equation. This step makes the left hand side of the equation a perfect square.

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

Square -\frac{5}{2} by squaring both the numerator and the denominator of the fraction.

$$x^{2}-5x+\frac{25}{4}=-100+\frac{25}{4}$$

Add -100 to \frac{25}{4}.

$$x^{2}-5x+\frac{25}{4}=-\frac{375}{4}$$

Factor x^{2}-5x+\frac{25}{4}. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.

$$\left(x-\frac{5}{2}\right)^{2}=-\frac{375}{4}$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x-\frac{5}{2}\right)^{2}}=\sqrt{-\frac{375}{4}}$$

Simplify.

$$x-\frac{5}{2}=\frac{5\sqrt{15}i}{2} x-\frac{5}{2}=-\frac{5\sqrt{15}i}{2}$$

Add \frac{5}{2} to both sides of the equation.

$$x=\frac{5+5\sqrt{15}i}{2} x=\frac{-5\sqrt{15}i+5}{2}$$