I Understanding Double Negatives in Algebraic Expressions

[dankMath]

An assignment of mine was returned with point deductions for writing:
"-1.x- - y.x" and was told it must be written "-1.x -(-y).x"
I asked about it and was told you can't have double negatives in working. I lost approximately 20% worth of the question, despite having perfect working for two other pages.

Is this a valid point?

 

[jedishrfu]

Welcome to PF!

In grading papers, it's always the teacher's discretion as to what grade you get. We don't know if he/she had emphasized this point in class or not which would make it appropriate to take away points.

You could ask and I mean ask him/her about it because I'm sure many of us have written this type of expression during an intermediate step for bookkeeping and then properly adjusted it as we continue to solve the problem. A key point of argument here too is whether you got the problem right and you showed you understood that it's poor form to show double negatives but that you did in fact solve it properly.

The use of parentheses is usually for clarity as shown in this case or to clearly indicate the order of operations.

Would he/she have objected to other notations like x * -3 = -3x ?

Having said all this, in the end it's always the teacher's discretion and you need to work within those bounds as you ask her for clarification so as not to do it on a test.

I sure hope this isn't a 5 point problem and you are arguing over a single point because then you are missing the point. We learn more from our mistakes than we do from our successes.

 

[olivermsun]

An assignment of mine was returned with point deductions for writing:
"-1.x- - y.x" and was told it must be written "-1.x -(-y).x"
I asked about it and was told you can't have double negatives in working. I lost approximately 20% worth of the question, despite having perfect working for two other pages.

Is this a valid point?

Generally speaking, I'd call it valid. Mitigating factors might include how the notation was covered in class and how your solution actually appeared, as written, on the page.

Since you had "perfect working" for the two other pages, did this turn out to be an issue re: your grade?

 

[dankMath]

Thanks for the reply!

It was not covered in class for any clarification. It was perfectly acceptable to have "-1 * x" or similar and my solution was correctly simplified. I was told that a double negative was simply not valid for working.
The reason I was asking about the question was to not complain regarding the points given, it's just every time I get an assignment back it's a new problem that has never been addressed until then. Although the assignments are somewhat minor, the same marking criteria is used for tests and exams which is slightly discouraging.

Regardless, thanks for the help :)

 

[jedishrfu]

I had some teachers who would nit pick my English even on a science or social studies paper which I thought as grossly unfair since it's not English class. But you eventually learn that every little defect in your writing detracts from its message and the same is true in math so go with the flow and learn these conventions from your teacher and it will make you a better and more precise student.

 

[olivermsun]

I agree, this kind of points loss is discouraging... and I sympathize with you.

That being said: -1 * x is generally acceptable, because the - is at the beginning and not part of a double negative.

More generally: What I suggest to you is to take this in stride and avoid leaving yourself open to these kinds of deductions in the future. If you know that you know what you're doing, then why leave it to chance?

 

[Bystander]

x * -3 = 3x

 

[jedishrfu]

thanks, fixed in post.

 

[Bystander]

" 'Sallright."

 

[Mark44]

An assignment of mine was returned with point deductions for writing:
"-1.x- - y.x"

If this was your final answer, I agree with the point deductions.
"-1x" should be written as -x, and " ... - - y * x" should also be simplified to "... + y * x"
I suspect that this was a problem in a beginning algebra class. As a general rule, your answers should be simplified as much as possible, unless different instructions are given.

On the other hand, if what you have above was the work leading to the final answer, then I think your teacher is being pedantic. There is no rule that says you can't have two or more negative signs in a row, although in an expression like this-- x - - y -- the first minus sign indicates a subtraction, and the second one indicates the negative of y.