Solve 25x^2+5x=20

$$25 { x }^{ 2 } +5x=20$$

Solve for x

$$x=-1$$

$$x=\frac{4}{5}=0.8$$

Steps Using Factoring By Grouping

Subtract 20 from both sides.

$$25x^{2}+5x-20=0$$

Divide both sides by 5.

$$5x^{2}+x-4=0$$

To solve the equation, factor the left hand side by grouping. First, left hand side needs to be rewritten as 5x^{2}+ax+bx-4. To find a and b, set up a system to be solved.

$$a+b=1 ab=5\left(-4\right)=-20$$

Since ab is negative, a and b have the opposite signs. Since a+b is positive, the positive number has greater absolute value than the negative. List all such integer pairs that give product -20.

$$-1,20 -2,10 -4,5$$

Calculate the sum for each pair.

$$-1+20=19 -2+10=8 -4+5=1$$

The solution is the pair that gives sum 1.

$$a=-4 b=5$$

Rewrite 5x^{2}+x-4 as \left(5x^{2}-4x\right)+\left(5x-4\right).

$$\left(5x^{2}-4x\right)+\left(5x-4\right)$$

Factor out x in 5x^{2}-4x.

$$x\left(5x-4\right)+5x-4$$

Factor out common term 5x-4 by using distributive property.

$$\left(5x-4\right)\left(x+1\right)$$

To find equation solutions, solve 5x-4=0 and x+1=0.

$$x=\frac{4}{5} x=-1$$

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.

$$25x^{2}+5x=20$$

Subtract 20 from both sides of the equation.

$$25x^{2}+5x-20=20-20$$

Subtracting 20 from itself leaves 0.

$$25x^{2}+5x-20=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute 25 for a, 5 for b, and -20 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$x=\frac{-5±\sqrt{5^{2}-4\times 25\left(-20\right)}}{2\times 25}$$

Square 5.

$$x=\frac{-5±\sqrt{25-4\times 25\left(-20\right)}}{2\times 25}$$

Multiply -4 times 25.

$$x=\frac{-5±\sqrt{25-100\left(-20\right)}}{2\times 25}$$

Multiply -100 times -20.

$$x=\frac{-5±\sqrt{25+2000}}{2\times 25}$$

Add 25 to 2000.

$$x=\frac{-5±\sqrt{2025}}{2\times 25}$$

Take the square root of 2025.

$$x=\frac{-5±45}{2\times 25}$$

Multiply 2 times 25.

$$x=\frac{-5±45}{50}$$

Now solve the equation x=\frac{-5±45}{50} when ± is plus. Add -5 to 45.

$$x=\frac{40}{50}$$

Reduce the fraction \frac{40}{50} to lowest terms by extracting and canceling out 10.

$$x=\frac{4}{5}$$

Now solve the equation x=\frac{-5±45}{50} when ± is minus. Subtract 45 from -5.

$$x=-\frac{50}{50}$$

Divide -50 by 50.

$$x=-1$$

The equation is now solved.

$$x=\frac{4}{5} x=-1$$

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.

$$25x^{2}+5x=20$$

Divide both sides by 25.

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

Dividing by 25 undoes the multiplication by 25.

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

Reduce the fraction \frac{5}{25} to lowest terms by extracting and canceling out 5.

$$x^{2}+\frac{1}{5}x=\frac{20}{25}$$

Reduce the fraction \frac{20}{25} to lowest terms by extracting and canceling out 5.

$$x^{2}+\frac{1}{5}x=\frac{4}{5}$$

Divide \frac{1}{5}, the coefficient of the x term, by 2 to get \frac{1}{10}. Then add the square of \frac{1}{10} to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$x^{2}+\frac{1}{5}x+\left(\frac{1}{10}\right)^{2}=\frac{4}{5}+\left(\frac{1}{10}\right)^{2}$$

Square \frac{1}{10} by squaring both the numerator and the denominator of the fraction.

$$x^{2}+\frac{1}{5}x+\frac{1}{100}=\frac{4}{5}+\frac{1}{100}$$

Add \frac{4}{5} to \frac{1}{100} by finding a common denominator and adding the numerators. Then reduce the fraction to lowest terms if possible.

$$x^{2}+\frac{1}{5}x+\frac{1}{100}=\frac{81}{100}$$

Factor x^{2}+\frac{1}{5}x+\frac{1}{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}.

$$\left(x+\frac{1}{10}\right)^{2}=\frac{81}{100}$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x+\frac{1}{10}\right)^{2}}=\sqrt{\frac{81}{100}}$$

Simplify.

$$x+\frac{1}{10}=\frac{9}{10} x+\frac{1}{10}=-\frac{9}{10}$$

Subtract \frac{1}{10} from both sides of the equation.

$$x=\frac{4}{5} x=-1$$