Solve 0.2x^2+4.3x-8.6=0

$$0.2 x ^ { 2 } + 4.3 x - 8.6 = 0$$

Solve for x

$$x = \frac{\sqrt{2537} - 43}{4} \approx 1.842160259$$

$$x=\frac{-\sqrt{2537}-43}{4}\approx -23.342160259$$

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.

$$0.2x^{2}+4.3x-8.6=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute 0.2 for a, 4.3 for b, and -8.6 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-4.3±\sqrt{4.3^{2}-4\times 0.2\left(-8.6\right)}}{2\times 0.2}$$

Square 4.3 by squaring both the numerator and the denominator of the fraction.

$$x=\frac{-4.3±\sqrt{18.49-4\times 0.2\left(-8.6\right)}}{2\times 0.2}$$

Multiply -4 times 0.2.

$$x=\frac{-4.3±\sqrt{18.49-0.8\left(-8.6\right)}}{2\times 0.2}$$

Multiply -0.8 times -8.6 by multiplying numerator times numerator and denominator times denominator. Then reduce the fraction to lowest terms if possible.

$$x=\frac{-4.3±\sqrt{18.49+6.88}}{2\times 0.2}$$

Add 18.49 to 6.88 by finding a common denominator and adding the numerators. Then reduce the fraction to lowest terms if possible.

$$x=\frac{-4.3±\sqrt{25.37}}{2\times 0.2}$$

Take the square root of 25.37.

$$x=\frac{-4.3±\frac{\sqrt{2537}}{10}}{2\times 0.2}$$

Multiply 2 times 0.2.

$$x=\frac{-4.3±\frac{\sqrt{2537}}{10}}{0.4}$$

Now solve the equation x=\frac{-4.3±\frac{\sqrt{2537}}{10}}{0.4} when ± is plus. Add -4.3 to \frac{\sqrt{2537}}{10}.

$$x=\frac{\sqrt{2537}-43}{0.4\times 10}$$

Divide \frac{-43+\sqrt{2537}}{10} by 0.4 by multiplying \frac{-43+\sqrt{2537}}{10} by the reciprocal of 0.4.

$$x=\frac{\sqrt{2537}-43}{4}$$

Now solve the equation x=\frac{-4.3±\frac{\sqrt{2537}}{10}}{0.4} when ± is minus. Subtract \frac{\sqrt{2537}}{10} from -4.3.

$$x=\frac{-\sqrt{2537}-43}{0.4\times 10}$$

Divide \frac{-43-\sqrt{2537}}{10} by 0.4 by multiplying \frac{-43-\sqrt{2537}}{10} by the reciprocal of 0.4.

$$x=\frac{-\sqrt{2537}-43}{4}$$

The equation is now solved.

$$x=\frac{\sqrt{2537}-43}{4} x=\frac{-\sqrt{2537}-43}{4}$$

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.

$$0.2x^{2}+4.3x-8.6=0$$

Add 8.6 to both sides of the equation.

$$0.2x^{2}+4.3x-8.6-\left(-8.6\right)=-\left(-8.6\right)$$

Subtracting -8.6 from itself leaves 0.

$$0.2x^{2}+4.3x=-\left(-8.6\right)$$

Subtract -8.6 from 0.

$$0.2x^{2}+4.3x=8.6$$

Multiply both sides by 5.

$$\frac{0.2x^{2}+4.3x}{0.2}=\frac{8.6}{0.2}$$

Dividing by 0.2 undoes the multiplication by 0.2.

$$x^{2}+\frac{4.3}{0.2}x=\frac{8.6}{0.2}$$

Divide 4.3 by 0.2 by multiplying 4.3 by the reciprocal of 0.2.

$$x^{2}+21.5x=\frac{8.6}{0.2}$$

Divide 8.6 by 0.2 by multiplying 8.6 by the reciprocal of 0.2.

$$x^{2}+21.5x=43$$

Divide 21.5, the coefficient of the x term, by 2 to get 10.75. Then add the square of 10.75 to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$x^{2}+21.5x+10.75^{2}=43+10.75^{2}$$

Square 10.75 by squaring both the numerator and the denominator of the fraction.

$$x^{2}+21.5x+115.5625=43+115.5625$$

Add 43 to 115.5625.

$$x^{2}+21.5x+115.5625=158.5625$$

Factor x^{2}+21.5x+115.5625. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.

$$\left(x+10.75\right)^{2}=158.5625$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x+10.75\right)^{2}}=\sqrt{158.5625}$$

Simplify.

$$x+10.75=\frac{\sqrt{2537}}{4} x+10.75=-\frac{\sqrt{2537}}{4}$$

Subtract 10.75 from both sides of the equation.

$$x=\frac{\sqrt{2537}-43}{4} x=\frac{-\sqrt{2537}-43}{4}$$