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 #1
avatar+2446 
+1

Domain and range of the function \(y=\frac{1}{x}\)

 

Now, let's figure out the domain first. The domain of a function just means the possible range of x-values that output a real y-value.

 

Let's consider that. Is there any number for that would output a nonreal y-value?

 

Yes, 0. If you plug in 0 for x, you get a nonreal y-value. Will any others? No. I've made a table of the first few domain options.

 

Possible Domain Corresponding y-value Is it real?
-5 \(\frac{1}{-5}\) Yes
-4 \(\frac{1}{-4}\) Yes
-3 \(\frac{1}{-3}\) Yes
-2 \(\frac{1}{-2}\) Yes
-1 \(\frac{1}{-1}\) Yes
0 \(\frac{1}{0}\) No, a number divided by 0 is undefined.
1 \(\frac{1}{1}\) Yes
2 \(\frac{1}{2}\) Yes
3 \(\frac{1}{3}\) Yes
4 \(\frac{1}{4}\) Yes
5 \(\frac{1}{5}\) Yes

 

You can see that any numer, except 0, will output a real y-value. Therefore, the domain is any number that isn't 0. 

 

Domain: \(\{x\in {\rm I\!R}\hspace{1mm}: x\neq0\}\)

 

In other words, the domain is apart of the real numbers except 0.

 

Let's think about the range, too. The range is set of all possible values that the function can actually produce. Let's try making a table for that:

 

Possible Range Corresponding x-value Is it real?
-3 \(-\frac{1}{3}\) Yes
-2 \(-\frac{1}{2}\) Yes
-1 \(-1\) Yes
0 No solution No
1 \(1\) Yes
2 \(\frac{1}{2}\) Yes
3 \(\frac{1}{3}\) Yes

 

You might be wondering why 0 has "no solution" on it. Well, let's try and solve it:

 

\(y=\frac{1}{x}\) Substitute in 0 for y.
\(0=\frac{1}{x}\) Multiply my on both sides.
\(0=1\) This is a nonsensical answer. Since \(0\neq1\), there is no solution. Therefore, 0 cannot be a possible range value.
   

 

Any other real number works, so our range is:
 

\(\{f(x)\in {\rm I\!R}\hspace{1mm}:f(x)\neq0\}\)

 

To recap, the domain and range is as follows

 

Domain: \(\{x\in {\rm I\!R}\hspace{1mm}: x\neq0\}\)

Range: \(\{f(x)\in {\rm I\!R}\hspace{1mm}:f(x)\neq0\}\)

 

To answer your second question about what does \(\{x|x\leq0\}\) mean. It means that x is a member of the real numbers such that x is nonnegative (0, 2/9, 1, π, 4.83333..., etc.). However, x has to be nonnegative, so it can't be -3/4, -5, -8.980333333..., etc.

 #14
avatar+893 
+1

I'm with Melody.

 

For the 20 ball first part, the number of three ball combinations is 20C3 = 1140.

The number of ways in which we miss with all three numbers is 17C3 = 680.

The number of ways in which we have 1 hit and three misses is 3*17C2 = 408.

The number of ways in which we have 2 hits and 1 miss is 3C2*17 = 51.

The number of ways in which we have three hits is 1.

(Check that all possibilities have been covered, 680 + 408 + 51 + 1 = 1140.)

That gets us the probabilities,

zero hits 680/1140 = 0.59649 (5 dp),

1 hit 408/ 1140       = 0.35789 (5 dp),

2 hits 51/1140        = 0.04474 (5 dp),

3 hits 1/1140          = 0.00088 (5 dp).

 

An alternative method is to calculate the probabilities directly.

Using H for a hit and M for a miss, the probability of three misses MMM is (17/20)*(16/19)*(15/18) = 0.59649.

The probability of the sequence HMM is (3/20)*(17/19)*(16/18) = 0.119298, and since both of the other 1 hit sequences MHM and MMH have the same probability, the probability of 1 hit will be 3*0.119298 = 0.35789 (5 dp).

The probabilty of the two hit sequence HHM is (3/20)*(2/19)*(17/18) = 0.014912, and since both of the other two hit sequences HMH and MHH have the same probability, the probability of two hits will be 3*0.014912 = 0.04474 (5 dp).

The probability of three hits will be (3/20)*(2/19)*(1/18) = 0.00088 (5 dp).

 

That means that the probability of at least two hits (two matches) is 0.04474 + 0.00088 = 0.04562.

The probability of not winning that way will be 0.59649 + 0.35789 = 0.95438.

 

The probability of winning with the superball is 0.1 and not winning with the superball is 0.9.

 

There are four possible outcomes,

(1) not getting at least two matches and not picking the superball : probability 0.95438* 0.9 = 0.85894,

(2) getting at least two matches, but not the superball : probability                    0.04562*0.9 = 0.04106,

(3) not getting at least two matches but do pick the superball : probability        0.95438*0.1  = 0.09544,

(4) getting at least two matches and picking the superball : probability              0.04562*0.1 = 0.00456,

(the probabilities summing  to 1, as they should).

 

That means that the probability of winning is 0.04106 + 0.09544 + 0.00456 (or 1 - 0.85894) = 0.14106.

 

Tiggsy.

Jul 3, 2017
 #13
avatar+2236 
+1

Hi Melody:

 

Actually the difference in our solutions is significant; it’s not from rounding errors.

The main reason is 

\(\dfrac{52}{1140}+\dfrac{1}{10}\underbrace {- \dfrac{52}{11400}}_{\text{This Term}}\\\)

(My computer is not rendering any Latex. I will correct this display, if needed)

This subtracts 10% of the (2 of 3) ball wins.

I had initially thought of this but dismissed it because of the either/or nature of the win.

On reflection, I can see it does count the win twice when both the super ball and the white balls are a match. It should count one or the other but not both. The problem, now, is by subtracting 52/11400, it subtracts both wins.  So, what should the value be?

 

My quick answer is half that (26/11400).The reasoning is by subtracting half, then one or the other is subtracted but not both.

 

What thinkest thee, Dame Melody?


More comments:

I agree with the 52/1140, I neglected to include the 3 of 3 win in my equation, because I was patting my chimp head for figuring out Lancelot Link’s convoluted formula for counting a  Z subset of  matches of R in a  N choose R combination.  

 

Here are the adjusted numbers in the same format
6)  Add (1 / 1140) to (51 / 1140) giving (52 / 1140) for when all three numbers match.

 

Calculate number of times both the super ball and two of the three balls match
(52 / 1140)  * (1 / 10) = 52/11400      <---  ½ of this = (26 / 11400)

 

Subtract half of matches when both the super ball and two of the three balls match (This counts one or the other but not both.)

 

Calculate the harmonic average

H = 3 / ((52 / 1140) + (1 / 10) - (26 / 11400))  = 20.930232558

 

Equalized probability of each event (1 / 20.9023) * 3 (for the three events)
= (3 / 20.9023)
= (43 / 300)
= 0.143333…

Note: doing it this way is much easier (This is Melody’s method)

 ((52 / 1140) + (1 / 10) - (26 / 11400)) = (0.143333… )


Over all probability 14.33%

(Only the final calculation is rounded)


This question is interesting, but it seems very advanced.
Do you understand this math, Victoria?  


Comments and criticisms from mathematicians (and articulate trolls) are welcome.smiley

 

 

GA

Jul 3, 2017
Jul 2, 2017

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