Graph of function intersects line

In summary, a graph of function is a visual representation of the relationship between two variables, and a line intersecting it means they share at least one point on the coordinate plane. The point of intersection can be determined by solving the equations and represents the solution to both equations. This can be applied to solving real-world problems by representing variables as equations and finding their point of intersection.
  • #1
hopelesss
15
0
Explain why the graph of the function g (x) = x^3 - 2x + 1 intersects the line y = x in
each of the intervals (-2, -1), (0, 1) and (1, 2).

i hope someone can help me with this. To be honest i do not even know where to start.
i sketched g(x) and y=x and i don't see them intersect in the intervals... so i guess I am supposed to do something with g(x) first?
 
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  • #2
comma said:
Explain why the graph of the function g (x) = x^3 - 2x + 1 intersects the line y = x in
each of the intervals (-2, -1), (0, 1) and (1, 2).

i hope someone can help me with this. To be honest i do not even know where to start.
i sketched g(x) and y=x and i don't see them intersect in the intervals... so i guess I am supposed to do something with g(x) first?

The question is asking you to find where the lines of g(x) = x and you can verify that it is true here: Wolfram|Alpha: Computational Knowledge Engine

The question is asking you to find the roots of the cubic (i.e. \(\displaystyle g(x)=0\) and compare them to the line y=x

Supposing the three roots (it's a cubic so has 3 roots) are \(\displaystyle a_1\), \(\displaystyle a_2\) and \(\displaystyle a_3\) then these points are \(\displaystyle (a_1,a_1) \ ,\ (a_2,a_2) \ ,\ (a_3,a_3)\)

Do you know how to (try) to factor a cubic? If you need a clue note that \(\displaystyle g(1)=0\)

so \(\displaystyle (x-1)\) is a factor of g(x) which can be written as \(\displaystyle g(x) = (x-1)(Ax^2+Bx+C)\) where A, B and C are constants
 
  • #3
I would look at:

\(\displaystyle f(x)=g(x)-y=(x^3-2x+1)-(x)=x^3-3x+1\)

And then:

\(\displaystyle f'(x)=3x^2-3=3(x+1)(x-1)\)

We know then that $f$ has the following behavior:

Increasing on $(-\infty,-1)$

Decreasing on $(-1,1)$

Increasing on $(1,\infty)$

Now, we can compute:

\(\displaystyle f(-2)=-1\)

\(\displaystyle f(-1)=3\)

So, given the sign change of the function and its monotonic behavior on $(-2,-1)$, we know there is one root in that interval.

Can you complete the other two intervals?
 
  • #4
thanks for responding to my post
yes i think i know how to factor a cubic.
i use polynomial factoring and get x^3-2x+1=(x-1)(x^2+x-1).
But i see MarkFL takes g(x)-y, then takes f'(x) before he factorise
and i did not do that so then its wrong. :)

I don't understand how i can know f has the behavior MarkFL gives
 
  • #5
If a function, on some interval $(a,b)$, has a first derivative that does not change sign, then we know that function is strictly monotonic on that interval, that is, it either strictly increase or decreases. If we further find that the function has changed sign over that interval, then we know the function has 1 root somewhere in that interval. :D
 
  • #6
so when i plot (0,1) and (1,2) into x^3-3x+1 i get
f(0)=1 and f(1)=-1
f(1)=-1 and f(2)=3

so i now know i have one root in each interval? what do this mean for my task?
 
  • #7
comma said:
so when i plot (0,1) and (1,2) into x^3-3x+1 i get
f(0)=1 and f(1)=-1
f(1)=-1 and f(2)=3

so i now know i have one root in each interval? what do this mean for my task?

This means you have completed the task. :)
 
  • #8
I see i have much studying to do :)
thanks for all help, i will now know what to do if i get tasks like this again :)
 
  • #9
Just because we can (and as an exercise): ;)

The graphs of $y=g(x)$ and $y=x$:
\begin{tikzpicture}
%preamble \usepackage{pgfplots}
\begin{axis}[xmin=-3, xmax=3, ymin=-3, ymax=3]
\addplot[blue, thick, samples=101] (x, {x^3-2*x+1}) node at (-1.8,2) {$y=g(x)$};
\addplot[red, thick] (x, x) node at (-0.5,-1.2) {$y=x$};
\end{axis}
\end{tikzpicture}

The graph of $y=f(x)$ as defined by MarkFL.
\begin{tikzpicture}
%preamble \usepackage{pgfplots}
\begin{axis}[xmin=-3, xmax=3, ymin=-3, ymax=3]
\addplot[blue, thick, samples=101] (x, {x^3-3*x+1}) node at (-2.2,2) {$y=f(x)$};
\addplot[red, thick] (x, 0) node at (-0.7,-0.3) {$y=0$};
\end{axis}
\end{tikzpicture}

And sign schemas of $f$ and $f'$, showing where the graph slopes up and down, and where we will find zeroes:

\begin{tikzpicture}

\draw (-3,2) node
{$f$} -- (3,2);
\draw (-2,1.9) node[below] {-2} -- (-2,2.1) node[above] {-1};
\draw (-1,1.9) node[below] {-1} -- (-1,2.1) node[above] {+3};
\draw ( 0,1.9) node[below] {0} -- ( 0,2.1) node[above] {+1};
\draw ( 1,1.9) node[below] {+1} -- ( 1,2.1) node[above] {-1};
\draw ( 2,1.9) node[below] {+2} -- ( 2,2.1) node[above] {+3};

\draw (-3,0) node
{$f'$} -- (3,0);
\node at (-2, 0.2) {$+$};
\draw (-1,-0.1) node[below] {-1} -- (-1,0.1) node[above] {0};
\node at (0, 0.2) {$-$};
\draw ( 1,-0.1) node[below] {+1} -- ( 1,0.1) node[above] {0};
\node at (2, 0.2) {$+$};

\end{tikzpicture}
We can see that we have 3 zeroes in respectively the intervals $(-2,-1), (0,1), (1,2)$.​
 

FAQ: Graph of function intersects line

What is a graph of function?

A graph of function is a visual representation of the relationship between two variables, where one variable is dependent on the other. It consists of points plotted on a coordinate plane, which are then connected by a line or curve.

What is a line intersecting a graph of function?

A line intersecting a graph of function means that the line and the graph of function share at least one point in common on the coordinate plane. This point represents the solution to the equation formed by the line and the function.

How can you determine the point of intersection between a line and a graph of function?

The point of intersection between a line and a graph of function can be determined by solving the equation formed by the line and the function. This is typically done by setting the two equations equal to each other and solving for the common variable.

What does the point of intersection represent in a graph of function and line?

The point of intersection represents the solution to the equation formed by the line and the function. It is the point where the two variables have the same value, and therefore, satisfy both equations simultaneously.

How can you use the point of intersection to solve real-world problems?

The point of intersection can be used to solve real-world problems by representing different variables in the problem as equations and finding their point of intersection. This can help determine the value of the variables that satisfy all the equations and provide a solution to the problem.

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