Solve 100x^2+18x+81=0

$$100 { x }^{ 2 } +18x+81 = 0$$

Solve for x (complex solution)

$$x=\frac{-9+27\sqrt{11}i}{100}\approx -0.09+0.895488693i$$

$$x=\frac{-27\sqrt{11}i-9}{100}\approx -0.09-0.895488693i$$

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.

$$100x^{2}+18x+81=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute 100 for a, 18 for b, and 81 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-18±\sqrt{18^{2}-4\times 100\times 81}}{2\times 100}$$

Square 18.

$$x=\frac{-18±\sqrt{324-4\times 100\times 81}}{2\times 100}$$

Multiply -4 times 100.

$$x=\frac{-18±\sqrt{324-400\times 81}}{2\times 100}$$

Multiply -400 times 81.

$$x=\frac{-18±\sqrt{324-32400}}{2\times 100}$$

Add 324 to -32400.

$$x=\frac{-18±\sqrt{-32076}}{2\times 100}$$

Take the square root of -32076.

$$x=\frac{-18±54\sqrt{11}i}{2\times 100}$$

Multiply 2 times 100.

$$x=\frac{-18±54\sqrt{11}i}{200}$$

Now solve the equation x=\frac{-18±54\sqrt{11}i}{200} when ± is plus. Add -18 to 54i\sqrt{11}.

$$x=\frac{-18+54\sqrt{11}i}{200}$$

Divide -18+54i\sqrt{11} by 200.

$$x=\frac{-9+27\sqrt{11}i}{100}$$

Now solve the equation x=\frac{-18±54\sqrt{11}i}{200} when ± is minus. Subtract 54i\sqrt{11} from -18.

$$x=\frac{-54\sqrt{11}i-18}{200}$$

Divide -18-54i\sqrt{11} by 200.

$$x=\frac{-27\sqrt{11}i-9}{100}$$

The equation is now solved.

$$x=\frac{-9+27\sqrt{11}i}{100} x=\frac{-27\sqrt{11}i-9}{100}$$

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.

$$100x^{2}+18x+81=0$$

Subtract 81 from both sides of the equation.

$$100x^{2}+18x+81-81=-81$$

Subtracting 81 from itself leaves 0.

$$100x^{2}+18x=-81$$

Divide both sides by 100.

$$\frac{100x^{2}+18x}{100}=-\frac{81}{100}$$

Dividing by 100 undoes the multiplication by 100.

$$x^{2}+\frac{18}{100}x=-\frac{81}{100}$$

Reduce the fraction \frac{18}{100} to lowest terms by extracting and canceling out 2.

$$x^{2}+\frac{9}{50}x=-\frac{81}{100}$$

Divide \frac{9}{50}, the coefficient of the x term, by 2 to get \frac{9}{100}. Then add the square of \frac{9}{100} to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$x^{2}+\frac{9}{50}x+\left(\frac{9}{100}\right)^{2}=-\frac{81}{100}+\left(\frac{9}{100}\right)^{2}$$

Square \frac{9}{100} by squaring both the numerator and the denominator of the fraction.

$$x^{2}+\frac{9}{50}x+\frac{81}{10000}=-\frac{81}{100}+\frac{81}{10000}$$

Add -\frac{81}{100} to \frac{81}{10000} by finding a common denominator and adding the numerators. Then reduce the fraction to lowest terms if possible.

$$x^{2}+\frac{9}{50}x+\frac{81}{10000}=-\frac{8019}{10000}$$

Factor x^{2}+\frac{9}{50}x+\frac{81}{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}.

$$\left(x+\frac{9}{100}\right)^{2}=-\frac{8019}{10000}$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x+\frac{9}{100}\right)^{2}}=\sqrt{-\frac{8019}{10000}}$$

Simplify.

$$x+\frac{9}{100}=\frac{27\sqrt{11}i}{100} x+\frac{9}{100}=-\frac{27\sqrt{11}i}{100}$$

Subtract \frac{9}{100} from both sides of the equation.

$$x=\frac{-9+27\sqrt{11}i}{100} x=\frac{-27\sqrt{11}i-9}{100}$$