Question

How do you solve $2{x^2} - x + 8 = 0$ ?

Answer 1

Hint: First compare the given quadratic equation to standard quadratic equation and find the value of numbers $a$, $b$ and $c$ in given equation. Then, substitute the values of $a$, $b$ and $c$ in the formula of discriminant and find the discriminant of the given equation. Finally, put the values of $a$, $b$ and $D$ in the roots of the quadratic equation formula and get the desired result.

Formula used:
The quantity $D = {b^2} - 4ac$ is known as the discriminant of the equation $a{x^2} + bx + c = 0$ and its roots are given by
$x = \dfrac{{ - b \pm \sqrt D }}{{2a}}$ or $x = \dfrac{{ - b \pm \sqrt {{b^2} - 4ac} }}{{2a}}$

Complete step by step answer:
We know that an equation of the form $a{x^2} + bx + c = 0$, $a,b,c,x \in R$, is called a Real Quadratic Equation.
The numbers $a$, $b$ and $c$ are called the coefficients of the equation.
The quantity $D = {b^2} - 4ac$ is known as the discriminant of the equation $a{x^2} + bx + c = 0$ and its roots are given by
$x = \dfrac{{ - b \pm \sqrt D }}{{2a}}$ or $x = \dfrac{{ - b \pm \sqrt {{b^2} - 4ac} }}{{2a}}$
So, first compare $2{x^2} - x + 8 = 0$ quadratic equation to standard quadratic equation and find the value of numbers $a$, $b$ and $c$.
Comparing $2{x^2} - x + 8 = 0$ with $a{x^2} + bx + c = 0$, we get
$a = 2$, $b = - 1$ and $c = 8$
Now, substitute the values of $a$, $b$ and $c$ in $D = {b^2} - 4ac$ and find the discriminant of the given equation.
$D = {\left( { - 1} \right)^2} - 4\left( 2 \right)\left( 8 \right)$
After simplifying the result, we get
$ \Rightarrow D = 1 - 64$
$ \Rightarrow D = - 63$
Which means the given equation has no real roots.
Now putting the values of $a$, $b$ and $D$ in $x = \dfrac{{ - b \pm \sqrt D }}{{2a}}$, we get
$x = \dfrac{{ - \left( { - 1} \right) \pm \sqrt {63} i}}{{2 \times 2}}$
It can be written as
$ \Rightarrow x = \dfrac{{1 \pm \sqrt {63} i}}{4}$
$ \Rightarrow x = \dfrac{1}{4} + \dfrac{{\sqrt {63} }}{4}i$ and $x = \dfrac{1}{4} - \dfrac{{\sqrt {63} }}{4}i$
So, $x = \dfrac{1}{4} + \dfrac{{\sqrt {63} }}{4}i$ and $x = \dfrac{1}{4} - \dfrac{{\sqrt {63} }}{4}i$ are roots/solutions of equation $2{x^2} - x + 8 = 0$.

Therefore, the solutions to the quadratic equation $2{x^2} - x + 8 = 0$ are $x = \dfrac{1}{4} + \dfrac{{\sqrt {63} }}{4}i$ and $x = \dfrac{1}{4} - \dfrac{{\sqrt {63} }}{4}i$..

Note: We can check whether $x = \dfrac{1}{4} + \dfrac{{\sqrt {63} }}{4}i$ and $x = \dfrac{1}{4} - \dfrac{{\sqrt {63} }}{4}i$ are roots/solutions of equation $2{x^2} - x + 8 = 0$ by putting the value of $x$ in given equation.
Putting $x = \dfrac{1}{4} + \dfrac{{\sqrt {63} }}{4}i$ in LHS of equation $2{x^2} - x + 8 = 0$.
\[{\text{LHS}} = 2{\left( {\dfrac{1}{4} + \dfrac{{\sqrt {63} }}{4}i} \right)^2} - \left( {\dfrac{1}{4} + \dfrac{{\sqrt {63} }}{4}i} \right) + 8\]
On simplification, we get
\[ \Rightarrow {\text{LHS}} = 2\left( {\dfrac{1}{{16}} - \dfrac{{63}}{{16}} + \dfrac{{\sqrt {63} }}{8}i} \right) - \dfrac{1}{4} - \dfrac{{\sqrt {63} }}{4}i + 8\]
\[ \Rightarrow {\text{LHS}} = - \dfrac{{31}}{4} + \dfrac{{\sqrt {63} }}{4}i + \dfrac{{31}}{4} - \dfrac{{\sqrt {63} }}{4}i\]
\[ \Rightarrow {\text{LHS}} = 0\]
$\therefore {\text{LHS}} = {\text{RHS}}$
Thus, $x = \dfrac{1}{4} + \dfrac{{\sqrt {63} }}{4}i$ is a solution of equation $2{x^2} - x + 8 = 0$.
Putting $x = \dfrac{1}{4} - \dfrac{{\sqrt {63} }}{4}i$ in LHS of equation $2{x^2} - x + 8 = 0$.
\[{\text{LHS}} = 2{\left( {\dfrac{1}{4} - \dfrac{{\sqrt {63} }}{4}i} \right)^2} - \left( {\dfrac{1}{4} - \dfrac{{\sqrt {63} }}{4}i} \right) + 8\]
On simplification, we get
\[ \Rightarrow {\text{LHS}} = 2\left( {\dfrac{1}{{16}} - \dfrac{{63}}{{16}} - \dfrac{{\sqrt {63} }}{8}i} \right) - \dfrac{1}{4} + \dfrac{{\sqrt {63} }}{4}i + 8\]
\[ \Rightarrow {\text{LHS}} = - \dfrac{{31}}{4} - \dfrac{{\sqrt {63} }}{4}i + \dfrac{{31}}{4} + \dfrac{{\sqrt {63} }}{4}i\]
\[ \Rightarrow {\text{LHS}} = 0\]
$\therefore {\text{LHS}} = {\text{RHS}}$
Thus, $x = \dfrac{1}{4} - \dfrac{{\sqrt {63} }}{4}i$ is a solution of equation $2{x^2} - x + 8 = 0$.
Final solution: Therefore, the solutions to the quadratic equation $2{x^2} - x + 8 = 0$ are $x = \dfrac{1}{4} + \dfrac{{\sqrt {63} }}{4}i$ and $x = \dfrac{1}{4} - \dfrac{{\sqrt {63} }}{4}i$.