Solve 2x ^ 2 + 3x - 20 = 0 | Equatix - Math Solver

Fractions & Decimals

Ratios & Proportions

Basic Number Theory

Integers & Absolute Values

Factors & Multiples

Prime Factorization

Basic Equations & Inequalities

Coordinate Plane & Graphing

Linear Equations & Inequalities

Quadratic Equations

Polynomials & Factoring

Functions & Graphs

Systems of Equations

Trigonometric Functions

Trigonometric Identities & Equations

Right Triangle Trigonometry

Graphing Trigonometric Functions

Limits & Continuity

Derivatives & Applications

Integrals & Applications

Series & Sequences

Multivariable Calculus

Functions & Graphs

Polynomial & Rational Functions

Exponential & Logarithmic Functions

Sequences & Series

Descriptive Statistics

Normal Distribution

Hypothesis Testing

Regression Analysis

Linear Programming

Sets & Counting

Eigenvalues & Eigenvectors

Linear Transformations

Atomic Structure

Thermochemistry

Example

3(r+2s)=2t-4

a⋅(b-2)=3b

3(r+2s)=2t-4

a⋅(b-2)=3b

Question

$$2 x ^ { 2 } + 3 x - 20 = 0$$

Solve for x

$x=-4$
$x = \frac{5}{2} = 2\frac{1}{2} = 2.5$

Steps Using Factoring By Grouping

Steps Using the Quadratic Formula

Steps for Completing the Square

Steps Using Direct Factoring Method

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 $2x^{2}+ax+bx-20$. To find $a$ and $b$, set up a system to be solved.

$$a+b=3$$ $$ab=2\left(-20\right)=-40$$

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 $-40$.

$$-1,40$$ $$-2,20$$ $$-4,10$$ $$-5,8$$

Calculate the sum for each pair.

$$-1+40=39$$ $$-2+20=18$$ $$-4+10=6$$ $$-5+8=3$$

The solution is the pair that gives sum $3$.

$$a=-5$$ $$b=8$$

Rewrite $2x^{2}+3x-20$ as $\left(2x^{2}-5x\right)+\left(8x-20\right)$.

$$\left(2x^{2}-5x\right)+\left(8x-20\right)$$

Factor out $x$ in the first and $4$ in the second group.

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

Factor out common term $2x-5$ by using distributive property.

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

To find equation solutions, solve $2x-5=0$ and $x+4=0$.

$$x=\frac{5}{2}$$ $$x=-4$$

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.

$$2x^{2}+3x-20=0$$

This equation is in standard form: $ax^{2}+bx+c=0$. Substitute $2$ for $a$, $3$ for $b$, and $-20$ for $c$ in the quadratic formula, $\frac{-b±\sqrt{b^{2}-4ac}}{2a}$.

$$x=\frac{-3±\sqrt{3^{2}-4\times 2\left(-20\right)}}{2\times 2}$$

Square $3$.

$$x=\frac{-3±\sqrt{9-4\times 2\left(-20\right)}}{2\times 2}$$

Multiply $-4$ times $2$.

$$x=\frac{-3±\sqrt{9-8\left(-20\right)}}{2\times 2}$$

Multiply $-8$ times $-20$.

$$x=\frac{-3±\sqrt{9+160}}{2\times 2}$$

Add $9$ to $160$.

$$x=\frac{-3±\sqrt{169}}{2\times 2}$$

Take the square root of $169$.

$$x=\frac{-3±13}{2\times 2}$$

Multiply $2$ times $2$.

$$x=\frac{-3±13}{4}$$

Now solve the equation $x=\frac{-3±13}{4}$ when $±$ is plus. Add $-3$ to $13$.

$$x=\frac{10}{4}$$

Reduce the fraction $\frac{10}{4}$ to lowest terms by extracting and canceling out $2$.

$$x=\frac{5}{2}$$

Now solve the equation $x=\frac{-3±13}{4}$ when $±$ is minus. Subtract $13$ from $-3$.

$$x=-\frac{16}{4}$$

Divide $-16$ by $4$.

$$x=-4$$

The equation is now solved.

$$x=\frac{5}{2}$$ $$x=-4$$

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 $x^{2}+bx=c$.

$$2x^{2}+3x-20=0$$

Add $20$ to both sides of the equation.

$$2x^{2}+3x-20-\left(-20\right)=-\left(-20\right)$$

Subtracting $-20$ from itself leaves $0$.

$$2x^{2}+3x=-\left(-20\right)$$

Subtract $-20$ from $0$.

$$2x^{2}+3x=20$$

Divide both sides by $2$.

$$\frac{2x^{2}+3x}{2}=\frac{20}{2}$$

Dividing by $2$ undoes the multiplication by $2$.

$$x^{2}+\frac{3}{2}x=\frac{20}{2}$$

Divide $20$ by $2$.

$$x^{2}+\frac{3}{2}x=10$$

Divide $\frac{3}{2}$, the coefficient of the $x$ term, by $2$ to get $\frac{3}{4}$. Then add the square of $\frac{3}{4}$ to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$x^{2}+\frac{3}{2}x+\left(\frac{3}{4}\right)^{2}=10+\left(\frac{3}{4}\right)^{2}$$

Square $\frac{3}{4}$ by squaring both the numerator and the denominator of the fraction.

$$x^{2}+\frac{3}{2}x+\frac{9}{16}=10+\frac{9}{16}$$

Add $10$ to $\frac{9}{16}$.

$$x^{2}+\frac{3}{2}x+\frac{9}{16}=\frac{169}{16}$$

Factor $x^{2}+\frac{3}{2}x+\frac{9}{16}$. In general, when $x^{2}+bx+c$ is a perfect square, it can always be factored as $\left(x+\frac{b}{2}\right)^{2}$.

$$\left(x+\frac{3}{4}\right)^{2}=\frac{169}{16}$$

Take the square root of both sides of the equation.

$$\sqrt{\left(x+\frac{3}{4}\right)^{2}}=\sqrt{\frac{169}{16}}$$

Simplify.

$$x+\frac{3}{4}=\frac{13}{4}$$ $$x+\frac{3}{4}=-\frac{13}{4}$$

Subtract $\frac{3}{4}$ from both sides of the equation.

$$x=\frac{5}{2}$$ $$x=-4$$

Steps Using Direct Factoring Method

Quadratic equations such as this one can be solved by a new direct factoring method that does not require guess work. To use the direct factoring method, the equation must be in the form $x^2+Bx+C=0$.This is achieved by dividing both sides of the equation by $2$

$$x ^ 2 +\frac{3}{2}x -10 = 0$$

Let $r$ and $s$ be the factors for the quadratic equation such that $x^2+Bx+C=(x−r)(x−s)$ where sum of factors $(r+s)=−B$ and the product of factors $rs = C$

$$r + s = -\frac{3}{2} $$ $$ rs = -10$$

Two numbers $r$ and $s$ sum up to $-\frac{3}{2}$ exactly when the average of the two numbers is $\frac{1}{2}*-\frac{3}{2} = -\frac{3}{4}$. You can also see that the midpoint of $r$ and $s$ corresponds to the axis of symmetry of the parabola represented by the quadratic equation $y=x^2+Bx+C$. The values of $r$ and $s$ are equidistant from the center by an unknown quantity $u$. Express $r$ and $s$ with respect to variable $u$.

$$r = -\frac{3}{4} - u$$ $$s = -\frac{3}{4} + u$$

To solve for unknown quantity $u$, substitute these in the product equation $rs = -10$

$$(-\frac{3}{4} - u) (-\frac{3}{4} + u) = -10$$

Simplify by expanding $(a -b) (a + b) = a^2 – b^2$

$$\frac{9}{16} - u^2 = -10$$

Simplify the expression by subtracting $\frac{9}{16}$ on both sides

$$-u^2 = -10-\frac{9}{16} = -\frac{169}{16}$$

Simplify the expression by multiplying $-1$ on both sides and take the square root to obtain the value of unknown variable $u$

$$u^2 = \frac{169}{16}$$ $$u = \pm\sqrt{\frac{169}{16}} = \pm \frac{13}{4} $$

The factors $r$ and $s$ are the solutions to the quadratic equation. Substitute the value of $u$ to compute the $r$ and $s$.

$$r =-\frac{3}{4} - \frac{13}{4} = -4$$ $$s = -\frac{3}{4} + \frac{13}{4} = 2.500$$