Intermediate Value Theorem

The idea behind the Intermediate Value Theorem is this:

intermediate value A to B

 

When we have two points connected by a continuous curve:

... then there will be at least one place where the curve crosses the line!

 

Well of course we must cross the line to get from A to B!

Now that you know the idea, let's look more closely at the details.

Continuous

The curve must be continuous ... no gaps or jumps in it.

Continuous is a special term with an exact definition in calculus, but here we will use this simplified definition:

pencilwe can draw it without lifting our pen from the paper

More Formal

Here is the Intermediate Value Theorem stated more formally:

intermediate value A to B crosses at (c,w)

 

When:

Then ...

... there must be at least one value c within [a, b] such that f(c) = w

 

In other words the function y = f(x) at some point must be w = f(c)

Notice that:

At Least One

intermediate value A to B crosses at 3 points

It also says "at least one value c", which means we could have more.

Here, for example, are 3 points where f(x)=w:

How Is This Useful?

Whenever we can show that:

we can then safely say "yes, there is a value somewhere in between that is on the line".

Example: is there a solution to x5 - 2x3 - 2 = 0 between x=0 and x=2?

At x=0:

05 - 2 × 03 - 2 = -2

 

At x=2:

25 - 2 × 23 - 2 = 14

Now we know:

And, being a polynomial, the curve will be continuous,

so somewhere in between the curve must cross through y=0

 

Yes, there is a solution to x5 - 2x3 - 2 = 0 in the interval [0, 2]

An Interesting Thing!

The Intermediate Value Theorem Can Fix a Wobbly Table

intermediate value wobbly table

If your table is wobbly because of uneven ground ...

... just rotate the table to fix it!

The ground must be continuous (no steps such as poorly laid tiles).

Why does this work?

We can always have 3 legs on the ground, it is the 4th leg that is the trouble.

Imagine we are rotating the table, and the 4th leg could somehow go into the ground (like sand):

So there must be some point where the 4th leg perfectly touches the ground and the table won't wobble.

(The famous Martin Gardner wrote about this in Scientific American. There is also a very complicated proof somewhere).

Another One

country walk
At some point during a round-trip you will be
exactly as high as where you started.

(It only works if you don't start at the highest or lowest point.)

The idea is:

So there must be a point in between where you are exactly as high as where you started.

Oh, and your path must be continuous, no disappearing and reappearing somewhere else.

The same thing happens with temperature, pressure, and so on.

And There's More!

If you follow a circular path ... somewhere on that circle there will be points that are:

intermediate value circular path opposites
two points that are
directly opposite and at same height

 

Can you think of more examples?