If f(x) = 2x-5x^2 and g(x) = 5x+2, what is f(4)?

[ilovemath10]

If f(x) = 2x-5x^2 and g(x) = 5x+2, what is f(4)?

Possible answers include -2376, -1584, -362, -50, or none of these.

 

[Deleted member 4993]

Possible answers include -2376, -1584, -362, -50, or none of these.

[h=2]If f(x) = 2x-5x^2 and g(x) = 5x+2, what is f(4)?What are your thoughts?[/h]
Please share your work with us ...even if you know it is wrong.

If you are stuck at the beginning tell us and we'll start with the definitions.

You need to read the rules of this forum. Please read the post titled "Read before Posting" at the following URL:

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[ilovemath10]

[h=2]If f(x) = 2x-5x^2 and g(x) = 5x+2, what is f(4)?What are your thoughts?[/h]
Please share your work with us ...even if you know it is wrong.

If you are stuck at the beginning tell us and we'll start with the definitions.

You need to read the rules of this forum. Please read the post titled "Read before Posting" at the following URL:

http://www.freemathhelp.com/forum/announcement.php?f=33

I need to know the answer now out of necessity, but would like to learn how to do that type of problem.

 

[Dr.Peterson]

If f(x) = 2x-5x^2 and g(x) = 5x+2, what is f(4)?
Possible answers include -2376, -1584, -362, -50, or none of these.

If you are unfamiliar with the notation involved here, you can read about it here.

After doing that (or checking your textbook), please write back and show what you have tried, so we can see what help you need.

I have to say that the choices offered are not what I would expect (the right answer, and several natural mistakes a student could make).

 

[ilovemath10]

If you are unfamiliar with the notation involved here, you can read about it here.

After doing that (or checking your textbook), please write back and show what you have tried, so we can see what help you need.

I have to say that the choices offered are not what I would expect (the right answer, and several natural mistakes a student could make).

My trouble is what to do with the problem. I plug in 4 for x in both equations, and end up with -72 on the one with f, and 22 for g. Now what? I don't understand what I'm supposed to do now.

 

[ilovemath10]

If you are unfamiliar with the notation involved here, you can read about it here.

After doing that (or checking your textbook), please write back and show what you have tried, so we can see what help you need.

I have to say that the choices offered are not what I would expect (the right answer, and several natural mistakes a student could make).

I begin by plugging in 4 for x in the first equation, and end up with -72. If I remember these kinds of problems correctly, aren't you supposed to plug in your answer from the first equation into the second equation or something?

 

[Deleted member 4993]

If f(x) = 2x-5x^2 and g(x) = 5x+2, what is f(4)?

Possible answers include -2376, -1584, -362, -50, or none of these.

Are you sure that you have posted the given problem completely and accurately? Please confirm.

 

[ilovemath10]

Are you sure that you have posted the given problem completely and accurately? Please confirm.

Yes, it is in it's complete and legitimate form. Can someone solve it?

 

[Dr.Peterson]

I begin by plugging in 4 for x in the first equation, and end up with -72. If I remember these kinds of problems correctly, aren't you supposed to plug in your answer from the first equation into the second equation or something?

No, you just do what you are told to do!

In this case (assuming you quoted the problem correctly), you were told only to evaluate f(4), so you can totally ignore g. That is probably there for a different problem in the same set.

I, too, get f(4) = -72, so the answer is "none of these". Unless, again, you didn't copy the problem correctly.

There will be other problems in which you might have to do composition of functions (which is what you are thinking of), but this is not that problem.

 

[ilovemath10]

No, you just do what you are told to do!

In this case (assuming you quoted the problem correctly), you were told only to evaluate f(4), so you can totally ignore g. That is probably there for a different problem in the same set.

I, too, get f(4) = -72, so the answer is "none of these". Unless, again, you didn't copy the problem correctly.

There will be other problems in which you might have to do composition of functions (which is what you are thinking of), but this is not that problem.

Thanks. Could you also teach me how to do these composition functions you speak of?

 

[Steven G]

Possible answers include -2376, -1584, -362, -50, or none of these.

Hi. Functions just like people are given names. If you were given two names of your new neighbor, epsilon and delta, and then were told something about delta, then you would not have a problem knowing that you were told something about delta.

You are asked to find something about the function call f so we ignore f, We compute f(4) using the f function and get -72

 

[Steven G]

Thanks. Could you also teach me how to do these composition functions you speak of?

By definition, the fog(x) = f(g(x).
This is saying to first find g(x) and then find f of that result.

So for example to find fog(4) you 1st find g(4) then find f of what ever g(4) happens to be.

Using your formulas for f and g can you find fog(x) and fog(4)?

 

[ilovemath10]

I'm honestly so happy I found this website. Up until now, I've been using Yahoo answers and quora for my math troubles, and you guys answered a question in 30 minutes when quora hasn't answered it in a week. You guys rock!

 

[Steven G]

I'm honestly so happy I found this website. Up until now, I've been using Yahoo answers and quora for my math troubles, and you guys answered a question in 30 minutes when quora hasn't answered it in a week. You guys rock!

I'm glad that you like this site! Now please tell us what fog(x) and fog(x) equals.

 

[ilovemath10]

I'm glad that you like this site! Now please tell us what fog(x) and fog(x) equals.

Huh?

 

[lookagain]

Thanks. Could you also teach me how to do these composition functions you speak of?

This is not a teaching site.

You are to learn that in the classroom. Tutors/helpers are to coach/reinforce what you have already gone over and have questions
over and/or shown some attempts here.

 

[Dr.Peterson]

Thanks. Could you also teach me how to do these composition functions you speak of?

Here is one place to go for this kind of help. The third page in will be most directly relevant, but you should read through the whole presentation. Here is another.