Would you please tell me what limits are and how you find them?

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- Aug 8th 2006, 07:17 PM #16

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Originally Posted by**Quick**

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There are 3 types systems involving linear equations: inconsistent, consistent and dependent and consistent and independent.

**Inconsistent**: If you are given two equations that are contradictory there are no solutions. For example,

How can two numbers add to 1 and 2? So there are no solutions. But I want you to look at is determinant of its coefficient.

. It is zero.

**Consistent and Dependant**. This is a system that has infinitely many solutions. For example,

Because the second equation can be divided by two to give,

which is the same as the first. Thus for any value of you can find a . Thus, there are infinitely many solutions.

Now, find the determinat of the coefficients.

. Also, zero.

**Warning**When you find the determinant of the system you need the variables to be**lined up**. For example,

. You need to line up the x's the y's and the z's*and then*take the determinant. Thus, it is,

**Consistent and Independant**. This is a system with a*unique*solution. For example,

.

Its determinant of the coefficients is,

. This leads to the following theorem.

**Theorem**A system of linear equations is consistent and independant (unique solution) only when, its determinant is N0N-ZER0.

Of course, when the determinant is zero it can have no solutions (inconsistent) or infinitely many solutions (consistent and dependant). However, how can you determine which one it is when determinant is zero? Later on I can show you.

**Definition**A system of linear equations,

is called*homogenous*.

**Theorem**If a homogenous system of linear equations has its determinant zero then it has infinitely many solutions.

**Proof**When the determinant of the system

Then it can have either infintely many solution or no solutions (it cannot have a unique solution). But this equation does have solution, namely the*trivial*,

Thus, it CANNOT have no solutions, thus it must have infinitely many solutions. (This type of equation does appear in applied math often and it is the case when the determinat is zero because otherwise it is not interesting).

Hope you like this lecture, that should entertain you for some hours. Next, time I will show you how to solve equations with determinants..

- Aug 13th 2006, 04:18 AM #17

- Aug 13th 2006, 10:23 AM #18

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This discussion would be mathematically limited (get the pun) for the sake of simplicity.

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Assume, you have a function, and you wish to find,

. What it means is what values does approach as approaches ? Note, the function does not need to be defined at . I might be mistaken but I believe Pierre de Fermat (1601-1665) was the first person to use limits at a point. [Though the method of exhaustion was a favorite of Archimedes he did not compute it for points].

There are 3 ways to compute.

1)**Graphical**. This is a useful way to observe how the function behaves as it approaches a certain point.

2)**Numerical**. This is often the easiet method to use by creating a table of values.

3)**Analytical**. This method is the one which employs the use of theorems to deduce limits. Note, this is the only method acceptable for proofs. Remember, mathematicians do not accept graph (and do not use them) and they do not rely on numerical evidence.

The first method you probably already understand. Simply draw your graph and see how it behaves. Sometimes it might be difficult to draw a graph so it is easier to create a table of values. Before I procede to tabular method I will first define left and right limits.

There are two different types of limits.

. They respectively are called,*limit from the right*and the*limit from the left*. What it means is basic. You approach your point from the right, now from the right means all point larger than that point. And from the left means all point lesser than that point.

For example, compute the following limit,

?

Create a table of values getting closer**from the right**(but never reaching) and see how they affect the function.

it is resonable to say the limit from the right is 1.

Now from the left, meaning,

?

Create a table of values getting closer**from the right**(but never reaching) and see how they affect the function.

it is resonable to say the limit from the left is -1.

Note, an important fact.

.

That means, that the limit,

**does not exist**. Because from one side it is one value and from another side it is a different value. There is even a theorem in calculus (analysis).

**Theorem.**A limit of a function exists at a point if and only if the left and right limits*coincide*.

Now for an excerise create a table of values (from both sides) and say whether this limit exists (and what is its value)?

?

-=WARNING=-I do not want you to find since its limit is L then because you cannot substitute.

More about limits coming ahead.

- Aug 14th 2006, 07:38 AM #19

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- Aug 14th 2006, 09:42 AM #20

- Aug 14th 2006, 09:46 AM #21

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- Aug 14th 2006, 10:07 AM #22

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Discussion on limits will now countinue (get the pun).

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Whenever you calculate limits either they exist or they do not. When they do not exist there is a special case which should be noted.

**Defintion**. When the limit of a function at a point increases without bound, we say the limit is .**Warning**: It*does not*mean that the limit exits. It does not. It rather means it is a special case when the limit keeps increasing. Furthermore, this**is not**a number, it is a notation used to demonstrate this instance.

**Definition**. When the limit of a function at a point decreases without bound, we say that the limit is .**Warning**: It*does not*mean that he limit exists. It does not. It rather means it is a special case when the limit keeps descreasing. Furthermore, this**is not**a number, it is a notation used to demonstrate this instance.

Here is a good example which shows both instances in one.

Consider the function, . I recommend to use that graphing program you have to graph it and see how it looks.

We will analyze the limits of . As before, we will use a table from both side. First from the right.

.

You can see that the function just keeps getting bigger and bigger. Thus, we write,

Now from the left,

You can see that the function just keeps getting smaller and smaller. Thus, we write,

The graph will show the curve goes to the heavens from the right and goes to the underworld from the left.

As, an excercise do the limits of at

- Aug 14th 2006, 10:25 AM #23
Before going on, I would like to thank you for taking the time to do this hacker

Originally Posted by**ThePerfectHacker**

from the left we get:

therefore:

from the right we get:

therefore:

You make it sound like, even though both limits are +infty, that no limit exists...

- Aug 14th 2006, 11:18 AM #24

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You did everything correct!. Except on the tables you forgot to mention minus signs from the left!

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Originally Posted by**Quick**

As, an additional point I would like to mention that sometimes you might want to write, (without left or right)

To indicate that both limits from the left and from the right are . The only advantage of this is that is saves writing space.

- Aug 14th 2006, 01:05 PM #25

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Before I get to theorems about limits there is still one more limit to learn. It is similar to the one previously but the positions are reversed. I am referring to,

. As you can guess it involves finding the limit as the number gets larger and larger. While the other one involves a number getting smaller and smaller.

For example,

Create a table of values, unlike the past there is no such thing as a limit from the left or right just simply make it smaller and smaller.

It is resonable to say the limit is one.

Thus, we write,

Another example, find

We find,

The limit does not exist but it increases without bound, thus,

When you calculate a limit at it can exist as in the first example or it cannot exists but increase/descrease without bound as in the second example. But can it be neither of these? Yes, observe (hope you know what radians are),

. I will create two tables of values,

and

Thus, take the function along odd multiplies of you get -1 (no matter how large) and take the function along even multiplies of you get 1 (no matther how large). Thus, the values*oscillate*between 1 and -1. Thus, the limit does not exist nor is .

This concludes the discussion on limits.

I will attach some example in the end of this post.

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This completes my discussion on limits. I will show some theorems involving limits. But before that, do the following examples (just post your solutions without the table).

Say whether the limits exists or they do not. If they do not say whether they are infinite limits or not. If they do say whether the value of the limits

1)

2)

3)

4)

5a) where in radians.

5b) where in degrees.

What conclusions can you make about 5a and 5b?

- Aug 17th 2006, 04:25 PM #26
1)

2)

3)

4)

5a) where in radians.

5b) where in degrees.

What conclusions can you make about 5a and 5b?

- Aug 17th 2006, 04:32 PM #27

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- Aug 17th 2006, 04:34 PM #28Originally Posted by
**ThePerfectHacker**

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The only thing I think I did wrong was set some limits at plus or minus infinity... (which I have changed in my post)

- Aug 17th 2006, 05:34 PM #29

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- Aug 25th 2006, 09:29 AM #30