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:

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

$$k=\left(-1*-100\pm sqrt\left(-{100}^2-4*0.09*-100\right)\right)/\left(2*0.09\right)$$

$$k=\left(-1*-100\pm sqrt\left(10000-4*0.09*-100\right)\right)/\left(2*0.09\right)$$

$$k=\left(-1*-100\pm sqrt\left(10000-0.36*-100\right)\right)/\left(2*0.09\right)$$

$$k=\left(-1*-100\pm sqrt\left(10000--36\right)\right)/\left(2*0.09\right)$$

$$k=\left(-1*-100\pm sqrt\left(10000+36\right)\right)/\left(2*0.09\right)$$

$$k=\left(-1*-100\pm sqrt\left(10036\right)\right)/\left(2*0.09\right)$$

$$k=\left(-1*-100\pm sqrt\left(10036\right)\right)/\left(0.18\right)$$

$$k=\left(100\pm sqrt\left(10036\right)\right)/0.18$$

to get the result:

$$k=\left(100\pm sqrt\left(10036\right)\right)/0.18$$

Simplify $$10036$$ by finding its prime factors:

The prime factorization of $$10036$$ is $$2^2\cdot 13\cdot 193$$

$$k=\left(100\pm 2*sqrt\left(2509\right)\right)/0.18$$

The ± means two roots are possible.

Separate the equations:
$$k_1=\left(100+2*sqrt\left(2509\right)\right)/0.18$$ and $$k_2=\left(100-2*sqrt\left(2509\right)\right)/0.18$$

$$k_1=\left(100+2*sqrt\left(2509\right)\right)/0.18$$

$$k_1=\left(100+2*sqrt\left(2509\right)\right)/0.18$$

$$k_1=\left(100+2*50.09\right)/0.18$$

$$k_1=\left(100+2*50.09\right)/0.18$$

$$k_1=\left(100+100.18\right)/0.18$$

$$k_1=\left(100+100.18\right)/0.18$$

$$k_1=\left(200.18\right)/0.18$$

$$k_1=\frac{200.18}{0.18}$$

$$k_1=1112.11$$

$$k_2=\left(100-2*sqrt\left(2509\right)\right)/0.18$$

$$k_2=\left(100-2*50.09\right)/0.18$$

$$k_2=\left(100-2*50.09\right)/0.18$$

$$k_2=\left(100-100.18\right)/0.18$$

$$k_2=\left(100-100.18\right)/0.18$$

$$k_2=\left(-0.18\right)/0.18$$

$$k_2=-\frac{0.18}{0.18}$$

$$k_2=-0.999$$

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: -0.999, 1112.11.

Since the $$a$$ coefficient is positive ($$a$$=0.09), 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 $$0.09k^2-100k-100<0$$ has a $$<$$ inequality sign, we look for the parabola intervals that are below the x-axis.

Solution: $$$$

Interval notation: $$$$