Why do I have to multiply by -1 when simplifying (sometimes)

In summary, the conversation discusses the process of solving for s in the equation -4su+7t+9u+2=6t+4u-9. It is clarified that the right-hand side of the equation has not been multiplied by -1, but rather factored out and canceled. The concept of multiplying by 1 is also explained, and it is noted that both \frac{-3}{-4} and \frac{3}{4} are equally simple.
  • #1
Jherek
4
0

Homework Statement



Solve for s:
[tex]-4su+7t+9u+2=6t+4u-9[/tex]

Homework Equations



The Attempt at a Solution


My answer:
[tex]s = \frac{-t-5u-11}{-4u}[/tex]
The given answer:
[tex]s=\frac{t+5u+11}{4u}[/tex]
I know that the right hand side has been multiplied by -1, but this raises some questions in my mind; Why do I have to do this, is my answer wrong and why isn't the left hand side also multiplied by -1?
Thanks for your help...
 
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  • #2
They haven't multiplied anything by -1. They factored -1 out from both the numerator and denominator and canceled it. Your answer is equivalent, but it's pretty typical to not leave any minus signs in denominators when simplifying.
 
  • #3
Ah. So that explains why the left-hand side is left untouched. It looks like the right hand side has been multiplied by -1 and that is how I've heard it explained before - but what you say makes more sense!
So I wasn't actually wrong as such, just not following convention.
Thank you.
 
  • #4
Jherek said:
Ah. So that explains why the left-hand side is left untouched. It looks like the right hand side has been multiplied by -1 and that is how I've heard it explained before
I suspect that what you were told is that on the right-hand side "we can multiply both "top" and "bottom" by -1". This is equivalent, if you think about it, to multiplying by 1 since (-1)/(-1) = 1. And when anything is multiplied by 1 you don't change its value. Hence, the left-hand side remains unchanged when you multiply the right-hand side by 1. (Remember, -1/-1 = 1)
 
  • #5
After all, which is simpler, [itex]\displaystyle \frac{\,-3}{\,-4}[/itex] or [itex]\displaystyle \frac{3}{4}\ ?[/itex]
 
  • #6
Well, I have to say that to me, they both look equally as simple! Especially if we were in the habit of writing [tex]\frac{+3}{+4}[/tex] which we don't so I'll just have to go along with what's expected!
 

1. Why do I have to multiply by -1 when simplifying?

When simplifying expressions, we often come across a situation where we need to get rid of a negative sign in front of a set of parentheses. In order to do this, we need to multiply the entire set of parentheses by -1. This is because multiplying a negative number by another negative number results in a positive number, effectively cancelling out the negative sign.

2. What is the purpose of multiplying by -1 when simplifying?

The purpose of multiplying by -1 when simplifying is to make the expression easier to work with and understand. By getting rid of negative signs, we can more clearly see the structure and relationships of the terms in the expression, making it easier to manipulate and solve.

3. Can I always multiply by -1 when simplifying?

Yes, you can always multiply by -1 when simplifying. This is because the distributive property of multiplication allows us to multiply any number by -1 without changing the value of the expression. So, no matter what the terms are in the parentheses, multiplying by -1 will always result in the same simplified expression.

4. Are there any exceptions to multiplying by -1 when simplifying?

There are two main exceptions to multiplying by -1 when simplifying. The first exception is when the expression already has a negative sign in front of it. In this case, we do not need to multiply by -1 as the negative sign is already accounted for. The second exception is when there is a coefficient or variable outside of the parentheses. In this case, we need to distribute the -1 to all terms inside the parentheses, which may result in a change in the expression.

5. Is there a specific order in which I should multiply by -1 when simplifying?

No, there is no specific order in which you should multiply by -1 when simplifying. As long as you distribute the -1 to all terms inside the parentheses, the order in which you do so does not matter. However, it is important to keep track of any coefficients or variables outside of the parentheses and make sure they are also multiplied by -1 if necessary.

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