Question

Zeroes of the polynomial $x^2-4x-21$ are :

• A. $3$ and $7$
• B. $-3$ and $7$
• C. $3$ and $-7$
• D. $-3$ and $-7$

Answer 1

The correct option is B $-3$ and $7$

We have, a formula for solving quadratic equation $a{x}^2+bx+c=0$ is

$x=\frac{-b\pm \sqrt{b^2-4ac}}{2a}$.

Zeroes or roots of the polynomial $x^2-4x-21$ are:

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

$=\frac{4\pm \sqrt{16+84}}{2}$

$=\frac{4+10}{2}$ and $\frac{4-10}{2}$

$=7$ and $-3$.

So, $B$ is correct.