Question

Which is the correct way to solve the given equation

Answer 1

Option 2. $x = \frac{4 \pm \sqrt{(-4)^{2}-4 (1)(-21)}}{2 (1)}$ shows the correct way to use the quadratic formula to solve the given equation.

Step-by-step explanation:

Step 1:

For an equation of the form $ax^{2} +bx+c=0$ the solution is $x=\frac{-b \pm \sqrt{b^{2}-4 a c}}{2 a}$.

Here a is the coefficient of $x^{2}$, b is the coefficient of x and c is the constant term.

$x^{2} -4x=21$ can also be written as $x^{2} -4x-21=0.$

Comparing $x^{2} -4x-21=0$ with $ax^{2} +bx+c=0$, we get that a is 1, b is -4 and c is -21.

To get the solution, we substitute the values of a, b, and c in $x=\frac{-b \pm \sqrt{b^{2}-4 a c}}{2 a}$.

Step 2:

Substituting the values, we get

$x=\frac{-b \pm \sqrt{b^{2}-4 a c}}{2 a}= \frac{-(-4) \pm \sqrt{(-4)^{2}-4 (1)(-21)}}{2 (1)}.$

$\frac{-(-4) \pm \sqrt{(-4)^{2}-4 (1)(-21)}}{2 (1)} = \frac{4 \pm \sqrt{(-4)^{2}-4 (1)(-21)}}{2 (1)}.$

This is option 2.