Solve -20x^2-64x+21=0

$$-20 { x }^{ 2 } -64x+21=0$$

Solve for x

$$x = -\frac{7}{2} = -3\frac{1}{2} = -3.5$$

$$x=\frac{3}{10}=0.3$$

Steps Using Factoring By Grouping

To solve the equation, factor the left hand side by grouping. First, left hand side needs to be rewritten as -20x^{2}+ax+bx+21. To find a and b, set up a system to be solved.

$$a+b=-64 ab=-20\times 21=-420$$

Since ab is negative, a and b have the opposite signs. Since a+b is negative, the negative number has greater absolute value than the positive. List all such integer pairs that give product -420.

$$1,-420 2,-210 3,-140 4,-105 5,-84 6,-70 7,-60 10,-42 12,-35 14,-30 15,-28 20,-21$$

Calculate the sum for each pair.

$$1-420=-419 2-210=-208 3-140=-137 4-105=-101 5-84=-79 6-70=-64 7-60=-53 10-42=-32 12-35=-23 14-30=-16 15-28=-13 20-21=-1$$

The solution is the pair that gives sum -64.

$$a=6 b=-70$$

Rewrite -20x^{2}-64x+21 as \left(-20x^{2}+6x\right)+\left(-70x+21\right).

$$\left(-20x^{2}+6x\right)+\left(-70x+21\right)$$

Factor out 2x in the first and 7 in the second group.

$$2x\left(-10x+3\right)+7\left(-10x+3\right)$$

Factor out common term -10x+3 by using distributive property.

$$\left(-10x+3\right)\left(2x+7\right)$$

To find equation solutions, solve -10x+3=0 and 2x+7=0.

$$x=\frac{3}{10} x=-\frac{7}{2}$$

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.

$$-20x^{2}-64x+21=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute -20 for a, -64 for b, and 21 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-\left(-64\right)±\sqrt{\left(-64\right)^{2}-4\left(-20\right)\times 21}}{2\left(-20\right)}$$

Square -64.

$$x=\frac{-\left(-64\right)±\sqrt{4096-4\left(-20\right)\times 21}}{2\left(-20\right)}$$

Multiply -4 times -20.

$$x=\frac{-\left(-64\right)±\sqrt{4096+80\times 21}}{2\left(-20\right)}$$

Multiply 80 times 21.

$$x=\frac{-\left(-64\right)±\sqrt{4096+1680}}{2\left(-20\right)}$$

Add 4096 to 1680.

$$x=\frac{-\left(-64\right)±\sqrt{5776}}{2\left(-20\right)}$$

Take the square root of 5776.

$$x=\frac{-\left(-64\right)±76}{2\left(-20\right)}$$

The opposite of -64 is 64.

$$x=\frac{64±76}{2\left(-20\right)}$$

Multiply 2 times -20.

$$x=\frac{64±76}{-40}$$

Now solve the equation x=\frac{64±76}{-40} when ± is plus. Add 64 to 76.

$$x=\frac{140}{-40}$$

Reduce the fraction \frac{140}{-40} to lowest terms by extracting and canceling out 20.

$$x=-\frac{7}{2}$$

Now solve the equation x=\frac{64±76}{-40} when ± is minus. Subtract 76 from 64.

$$x=-\frac{12}{-40}$$

Reduce the fraction \frac{-12}{-40} to lowest terms by extracting and canceling out 4.

$$x=\frac{3}{10}$$

The equation is now solved.

$$x=-\frac{7}{2} x=\frac{3}{10}$$

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.

$$-20x^{2}-64x+21=0$$

Subtract 21 from both sides of the equation.

$$-20x^{2}-64x+21-21=-21$$

Subtracting 21 from itself leaves 0.

$$-20x^{2}-64x=-21$$

Divide both sides by -20.

$$\frac{-20x^{2}-64x}{-20}=-\frac{21}{-20}$$

Dividing by -20 undoes the multiplication by -20.

$$x^{2}+\left(-\frac{64}{-20}\right)x=-\frac{21}{-20}$$

Reduce the fraction \frac{-64}{-20} to lowest terms by extracting and canceling out 4.

$$x^{2}+\frac{16}{5}x=-\frac{21}{-20}$$

Divide -21 by -20.

$$x^{2}+\frac{16}{5}x=\frac{21}{20}$$

Divide \frac{16}{5}, the coefficient of the x term, by 2 to get \frac{8}{5}. Then add the square of \frac{8}{5} to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$x^{2}+\frac{16}{5}x+\left(\frac{8}{5}\right)^{2}=\frac{21}{20}+\left(\frac{8}{5}\right)^{2}$$

Square \frac{8}{5} by squaring both the numerator and the denominator of the fraction.

$$x^{2}+\frac{16}{5}x+\frac{64}{25}=\frac{21}{20}+\frac{64}{25}$$

Add \frac{21}{20} to \frac{64}{25} by finding a common denominator and adding the numerators. Then reduce the fraction to lowest terms if possible.

$$x^{2}+\frac{16}{5}x+\frac{64}{25}=\frac{361}{100}$$

Factor x^{2}+\frac{16}{5}x+\frac{64}{25}. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.

$$\left(x+\frac{8}{5}\right)^{2}=\frac{361}{100}$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x+\frac{8}{5}\right)^{2}}=\sqrt{\frac{361}{100}}$$

Simplify.

$$x+\frac{8}{5}=\frac{19}{10} x+\frac{8}{5}=-\frac{19}{10}$$

Subtract \frac{8}{5} from both sides of the equation.

$$x=\frac{3}{10} x=-\frac{7}{2}$$