I encounter that quadratic inequality can have two ways of graphing, line(s) with the area of solutions and parts of a parabola. How is that happen?
1 Answer
This is similar to your previous question.
If you are looking for $x$ such that
\[x^2-4x\geq 14,\]
then
\[x^2-4x- 14\geq 0.\]
Finding the roots using the quadratic formula, you get
\[x_1,x_2=\frac{+4\pm\sqrt{14+56}}{2}=\frac{+4\pm\sqrt{70}}{2}.\]
Hence the inequality is satisfied if
\[ x\geq \frac{+4+\sqrt{70}}{2} \text{or} x\leq \frac{+4-\sqrt{70}}{2},\]
which are two line segments if graphed on $x$-axis.
However, if you view this problem as the graph of the function $y=x^2-4x- 14$, then you want the portion of the graph that lies above $x$-axis, i.e. $y=x^2-4x- 14\geq 0.$
So it is the same problem, viewed from two different points of view. The first view is one dimensional where we only have the variable $x$, while in the second case we view the problem in two dimensions and have two variables $x,y$.
Erdos
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