Solution - Solving quadratic inequalities using the quadratic formula

To find the roots of a quadratic equation, plug its coefficients ($$a$$, $$b$$ and $$c$$ ) into the quadratic formula:

$$x=\left(-b\pm sqrt\left(b^2-4ac\right)\right)/\left(2a\right)$$

$$x=\left(-1*-210\pm sqrt\left(-{210}^2-4*1*4000\right)\right)/\left(2*1\right)$$

$$x=\left(-1*-210\pm sqrt\left(44100-4*1*4000\right)\right)/\left(2*1\right)$$

$$x=\left(-1*-210\pm sqrt\left(44100-4*4000\right)\right)/\left(2*1\right)$$

$$x=\left(-1*-210\pm sqrt\left(44100-16000\right)\right)/\left(2*1\right)$$

$$x=\left(-1*-210\pm sqrt\left(28100\right)\right)/\left(2*1\right)$$

$$x=\left(-1*-210\pm sqrt\left(28100\right)\right)/\left(2\right)$$

$$x=\left(210\pm sqrt\left(28100\right)\right)/2$$

to get the result:

$$x=\left(210\pm sqrt\left(28100\right)\right)/2$$

Simplify $$28100$$ by finding its prime factors:

The prime factorization of $$28100$$ is $$2^2\cdot 5^2\cdot 281$$

$$x=\left(210\pm 10*sqrt\left(281\right)\right)/2$$

The ± means two roots are possible.

Separate the equations:
$$x_1=\left(210+10*sqrt\left(281\right)\right)/2$$ and $$x_2=\left(210-10*sqrt\left(281\right)\right)/2$$

$$x_1=\left(210+10*sqrt\left(281\right)\right)/2$$

$$x_1=\left(210+10*sqrt\left(281\right)\right)/2$$

$$x_1=\left(210+10*16.763\right)/2$$

$$x_1=\left(210+10*16.763\right)/2$$

$$x_1=\left(210+167.631\right)/2$$

$$x_1=\left(210+167.631\right)/2$$

$$x_1=\left(377.631\right)/2$$

$$x_1=\frac{377.631}{2}$$

$$x_1=188.815$$

$$x_2=\left(210-10*sqrt\left(281\right)\right)/2$$

$$x_2=\left(210-10*sqrt\left(281\right)\right)/2$$

$$x_2=\left(210-10*16.763\right)/2$$

$$x_2=\left(210-10*16.763\right)/2$$

$$x_2=\left(210-167.631\right)/2$$

$$x_2=\left(210-167.631\right)/2$$

$$x_2=\left(42.369\right)/2$$

$$x_2=\frac{42.369}{2}$$

$$x_2=21.185$$

To find the intervals of a quadratic inequality, we start by finding its parabola.

The roots of the parabola (where it meets the x-axis) are: 21.185, 188.815.

Since the $$a$$ coefficient is positive ($$a$$=1), this is a "positive" quadratic inequality and the parabola points upward, like a smile!

If the inequality sign is ≤ or ≥ , then the intervals include the roots and we use a solid line. If the inequality sign is < or > the intervals do not include the roots and we use a dotted line.

Since $$x^2-210x+4000>0$$ has a $$>$$ inequality sign, we look for the parabola intervals that are above the x-axis.

Solution: $$$$

Interval notation: $$$$