# Thread: Anyone know how to do this?

1. ## Anyone know how to do this?

Does anybody know how to do these kinds of math problems?:

"If f(x)=1+3x, find g(-2)."

2. ? school is over. why?

3. I'm pretty sure you plug -2 for "x" in the 1+3x and then solve it

4. that might work

5. The way you have it notated in your post, there isn't a way to find a solution. You haven't provided the relation that g(x) holds to f(x). g(x) needs to be defined in terms of f(x).
If your problem provides a basis of comparison - say, "f(x) = 4g(x)", then you can find g(x) by plugging variable x into f(x) and then quartering the result.

Give us the full problem and someone can show you step-by-step.

6. Originally Posted by BobR
The way you have it notated in your post, there isn't a way to find a solution. You haven't provided the relation that g(x) holds to f(x). g(x) needs to be defined in terms of f(x).
If your problem provides a basis of comparison - say, "f(x) = 4g(x)", then you can find g(x) by plugging variable x into f(x) and then quartering the result.

Give us the full problem and someone can show you step-by-step.
Yeah, I'm sorry, I wasn't to careful when I typed it. The problem is actually

"If f(x)=1+3x, find f(-4)." Sorry about the mixup.

7. f(x)=1+3(-4)
f(x)=1+-12
f(x)=1-12
f(x)=-11

8. can someone tell me why we need to know stuff like that, i have been out of school for 18 years and have yet too ever had a time when that would have come in handy

oh yeah, i do know how to do it either.

9. Originally Posted by savagenomore
can someone tell me why we need to know stuff like that
It a little thing called standardized tests - which translates to "A high-stakes test designed to test knowledge of problems that will be seen on the test, and only on the test; nowhere else in real life will you encounter these (unless you become a high-school math teacher)"

and functions are just a cryptic way of saying "y=1+3x, solve for y if x = -4".

10. Not quite true...

The stated problem is relatively basic algebra. That has many uses in real life (depending on what profession you decide to pursue). Some other things on the other hand, seem completely useless (*cough* taylor series *cough*)

11. Originally Posted by roboman2115
f(x)=1+3(-4)
f(x)=1+-12
f(x)=1-12
f(x)=-11
Ok, I tried a different problem following that example.

If g(x)=2x-5, find g(-2)
2(-2)-5
-4-5=-9

g(-2)=-9

Is that right?

12. Looks right to me...

13. Originally Posted by 2.5driver
Ok, I tried a different problem following that example.

If g(x)=2x-5, find g(-2)
2(-2)-5
-4-5=-9

g(-2)=-9

Is that right?
that's correct

14. Ok cool. Thanks everyone.

15. Heres another type of problem I am having troubles with.

-Find a number such that 12 more than four times the number is -56.

^^^How do you go about figureing that out.

-Travis and Judy thought of the same number. They multiplied the number by 2 and then decreased the product by 8 for a final result of 20. Of what number were they thinking.

^^^I know with that one, you could just think about it for a while and come up with the answer, but I know theres got to be an easier way to doing it. Any help would be great.

16. For the first one I got -26

For the second one, 20+8=28, 28/2=14

I know I'm probably wrong.... lol.

17. Originally Posted by 2.5driver
Heres another type of problem I am having troubles with.

-Find a number such that 12 more than four times the number is -56.

^^^How do you go about figureing that out.

-Travis and Judy thought of the same number. They multiplied the number by 2 and then decreased the product by 8 for a final result of 20. Of what number were they thinking.

^^^I know with that one, you could just think about it for a while and come up with the answer, but I know theres got to be an easier way to doing it. Any help would be great.
With these types of problems, you work backwards. Take the number they end up with, then go back and do the opposite operations in backwards order. For the first one, take -56, divide by 4 (leaving -14), then subtract 12 giving you -26. Do the same with the second, you should get 14. And about these problems being useless for in the future, my dad is a civil engineer. He uses problems that are exactly like these ones. Some examples would be in finding how much weight a type of soil could hold or how much stress a certain object can take. I thought at first it was useless, but then I saw what he was teaching and I realized how important it is.

18. [QUOTE=2.5driver]

-Travis and Judy thought of the same number. They multiplied the number by 2 and then decreased the product by 8 for a final result of 20. Of what number were they thinking.

QUOTE]

the answer to this is 14

14x2=28 - 8 = 20

19. Ok, so for this problem:

"Tom and Mike found that 4 times the sum of a number and 5 equals 136. What is the number?"

Would work backwards on that one to? If so how? There is no inverse operation for "=", so its not like I can do the opposite operation for that like I would on the other problems.

20. When it says it equals a number, it's the same as saying "a final result of ...". This one is a bit tricky though, first you divide 136 by 4 (34) then subtract 5, leaving 29. Another way to do these types of problems is to write them out as equations, with the "number" being x, ex. 4(x+5) = 136

"Kaye found that if the product of 4 and a number is decreased by 42, the result is 118 less than the product of 2 and the number. What is the number?"

Here is how I would set it up as an equation: 4(x-42)=118-2+x <<< I don't think that is right, so I thought I would ask.

Thanks a lot for all the help so far, it really has helped me. All these questions are coming out of a packet that I have to get done, and there are about 5 of each type of question, so this is really helping.

22. so, are we doing your homework for you or what..........lol

23. Solve that one with (4x)-42=2x-118

The reason for this is that the product of 4 and the number (x) is decreased by 42. Hence, (4x)-42. The 2x-118 is from it being 118 less than the product of 2 and the number (or, the product of 2 and the number minus one hundred eighteen).

If you want to solve this, then you take the original equation:
4x-42=2x-118

You move the variables to one side:
4x-42-2x=-118

You move the scalar quantities to the other:
4x-2x=-118+42

Combine like terms:
2x=-76

Solve for x:
x=-38

24. This is thread is beginning to sound like an SAT test. I don't want to think about those again, just got done taking them

25. heh...

I just finished my ACT a month ago, and got the results back a week or so ago. I know what you mean...

26. Originally Posted by enforcer
Solve that one with (4x)-42=2x-118

The reason for this is that the product of 4 and the number (x) is decreased by 42. Hence, (4x)-42. The 2x-118 is from it being 118 less than the product of 2 and the number (or, the product of 2 and the number minus one hundred eighteen).

If you want to solve this, then you take the original equation:
4x-42=2x-118

You move the variables to one side:
4x-42-2x=-118

You move the scalar quantities to the other:
4x-2x=-118+42

Combine like terms:
2x=-76

Solve for x:
x=-38
Thanks, I was just able to complete a few more problems following those steps.

I have yet another question. When you come across a number and it has an exponent of 0, does that make that number 0, or 1? On a differnt a problem, one of the numbers is 4(0) (the 0 in () is an exponent), and I don't know what that makes 4 now.

And no, I'm did not create this thread for you guys to do my homework for me. I have a very hard time with this, and at the moment this is my only means of finding out how to do this stuff. So far, it has helped tremendously because when I'm told how to do a problem, I can then go onto other problems that are just like it and do them.

27. What grade are you going into, becauce I did stuff like that in 7th grade (doesn't mean I know how to do it).
Oh yeah, I failed math this year. HEHE

28. I did that type of stuff when i was a freshmen.. Questions like that are on the SAT. Anything your school teaches you make sure you go home and make different problems so you can solve them. Practice makes perfect. I know when you drove your first r/c you wernt perfect it took some time, so practice. I specialize in math so shoot me a PM if you need any help in any work.

29. I'm going into 12th. Math is my worst subject. I think that if I would have kept up on it, I wouldn't be in summer school right now, and instead of going into geometry next year, I would be going into algebra 2.

Thanks for the help guys. I was able to get a lot done with your guys' help.

30. If your going into 12th grade, then you allready took the HSPA. How did you do on that.

I'm going into 10th, and I'm also doing summer school for math. I will be looking forward to summer school next year too, and the year after that.

31. Originally Posted by Dougherty
If your going into 12th grade, then you allready took the HSPA. How did you do on that.

I'm going into 10th, and I'm also doing summer school for math. I will be looking forward to summer school next year too, and the year after that.
I never took that test...

32. 2.5 - the key to problems like that to to filter out all the junk they put in it. All the stuff about judy and sally and frank and joe and jose, doesn't mean anything, so just delete it. for example, the important stuff is bolded, ignore everything else:

"Kaye found that if the product of 4 and a number is decreased by 42, the result is 118 less than the product of 2 and the number. What is the number?"

now, chop up the problem into each individual operation:

the product of 4 and a number (4x)

decreased by 42 (-42)

the result is (=)

118 less than (- 118) (whenever you see "less than" your subtracting the first number from the second. ex: "11 less than 5" = "5-11"

the product of 2 and the number (2x)

now you got each piece, now put it all together:

4x - 42 = 2x - 118

As you can see, the - 118 has been put behind the 2x since its "less than 2x" Now just solve as already show.

Also, a 0 exponent is always equal to 1, regardless of the base number.

33. X^0 is ALWAYS 1, regardless of x, except in one case.

0^0 is undefined.

All other numbers raised to the zeroth power equal 1.

Nice, simple rule

34. Thanks guys

35. Originally Posted by 2.5driver
I'm going into 12th. Math is my worst subject. I think that if I would have kept up on it, I wouldn't be in summer school right now, and instead of going into geometry next year, I would be going into algebra 2.
Hang in there guys. In the big picture it's not ganna matter that much whether you got this in 7th grade or Freshman year in college. Skool sux sometimes but if your into toys like RC cars, chances are you're pretty mechanical and possibly a natural engineer - which doesn't necessarily mean you're good in math. But if engineering/science is your bag, you're REALLY ganna need this. Unfortunately, it might not get that interesting until college. So hang in there.

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