Solve 5x^2-21x+14=1

$$5 { x }^{ 2 } -21x+14=1$$

Solve for x

$$x = \frac{\sqrt{181} + 21}{10} \approx 3.445362405$$

$$x=\frac{21-\sqrt{181}}{10}\approx 0.754637595$$

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.

$$5x^{2}-21x+14=1$$

Subtract 1 from both sides of the equation.

$$5x^{2}-21x+14-1=1-1$$

Subtracting 1 from itself leaves 0.

$$5x^{2}-21x+14-1=0$$

Subtract 1 from 14.

$$5x^{2}-21x+13=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute 5 for a, -21 for b, and 13 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-\left(-21\right)±\sqrt{\left(-21\right)^{2}-4\times 5\times 13}}{2\times 5}$$

Square -21.

$$x=\frac{-\left(-21\right)±\sqrt{441-4\times 5\times 13}}{2\times 5}$$

Multiply -4 times 5.

$$x=\frac{-\left(-21\right)±\sqrt{441-20\times 13}}{2\times 5}$$

Multiply -20 times 13.

$$x=\frac{-\left(-21\right)±\sqrt{441-260}}{2\times 5}$$

Add 441 to -260.

$$x=\frac{-\left(-21\right)±\sqrt{181}}{2\times 5}$$

The opposite of -21 is 21.

$$x=\frac{21±\sqrt{181}}{2\times 5}$$

Multiply 2 times 5.

$$x=\frac{21±\sqrt{181}}{10}$$

Now solve the equation x=\frac{21±\sqrt{181}}{10} when ± is plus. Add 21 to \sqrt{181}.

$$x=\frac{\sqrt{181}+21}{10}$$

Now solve the equation x=\frac{21±\sqrt{181}}{10} when ± is minus. Subtract \sqrt{181} from 21.

$$x=\frac{21-\sqrt{181}}{10}$$

The equation is now solved.

$$x=\frac{\sqrt{181}+21}{10} x=\frac{21-\sqrt{181}}{10}$$

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.

$$5x^{2}-21x+14=1$$

Subtract 14 from both sides of the equation.

$$5x^{2}-21x+14-14=1-14$$

Subtracting 14 from itself leaves 0.

$$5x^{2}-21x=1-14$$

Subtract 14 from 1.

$$5x^{2}-21x=-13$$

Divide both sides by 5.

$$\frac{5x^{2}-21x}{5}=-\frac{13}{5}$$

Dividing by 5 undoes the multiplication by 5.

$$x^{2}-\frac{21}{5}x=-\frac{13}{5}$$

Divide -\frac{21}{5}, the coefficient of the x term, by 2 to get -\frac{21}{10}. Then add the square of -\frac{21}{10} to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$x^{2}-\frac{21}{5}x+\left(-\frac{21}{10}\right)^{2}=-\frac{13}{5}+\left(-\frac{21}{10}\right)^{2}$$

Square -\frac{21}{10} by squaring both the numerator and the denominator of the fraction.

$$x^{2}-\frac{21}{5}x+\frac{441}{100}=-\frac{13}{5}+\frac{441}{100}$$

Add -\frac{13}{5} to \frac{441}{100} by finding a common denominator and adding the numerators. Then reduce the fraction to lowest terms if possible.

$$x^{2}-\frac{21}{5}x+\frac{441}{100}=\frac{181}{100}$$

Factor x^{2}-\frac{21}{5}x+\frac{441}{100}. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.

$$\left(x-\frac{21}{10}\right)^{2}=\frac{181}{100}$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x-\frac{21}{10}\right)^{2}}=\sqrt{\frac{181}{100}}$$

Simplify.

$$x-\frac{21}{10}=\frac{\sqrt{181}}{10} x-\frac{21}{10}=-\frac{\sqrt{181}}{10}$$

Add \frac{21}{10} to both sides of the equation.

$$x=\frac{\sqrt{181}+21}{10} x=\frac{21-\sqrt{181}}{10}$$