Math Is Fun Forum

shara

Member

Registered: 2014-01-13

Posts: 1

parenthesis question

Is it wrong not to use parenthesis before the minus that is before the number 5?

Do I HAVE TO write "-2 + (-5) = -7" and "-2 x (-5) = 10" or is it ok to write "-2 + -5 = -7" and "-2 x -5 = 10"?

Thanks,

Bob

Administrator

Registered: 2010-06-20

Posts: 10,145

Re: parenthesis question

hi shara

Welcome to the forum.

The easy answer is you can do either but read on for a more detailed account.

The words 'subtract', 'minus' and 'negative' are often used as if they meant the same thing and the symbol is the same.

But, strictly, they are different things.

Subtract is an operation that is applied to two numbers, giving a third number as the result.

eg.     6 subtract 4 = 2

Negative is a label that attaches to a number to indicate where it is on the number line (left of zero).

The use of brackets makes this clear.

-2 - (-5) = -7

(negative two) subtract (negative five).

If you leave out the brackets

-2 - -5 = -7

it is harder to spot that one of the - is 'subtract' and the other is 'negative'.

So I'd say it's a good idea to put in the brackets, but, many people don't bother.

I haven't said anything about 'minus' yet.  That's because it is carelessly used to mean either 'subtract' or 'negative'.

eg.  Two minuses make a plus really means 'if you multiply two negative numbers together the result is positive'

and 'if you subtract a negative number this is the same as adding the equivalent positive number'.

In practice, it's obviously a lot easier just to learn two minuses make a plus

But I have seen pupils make this mistake with the rule:

-3 + - 5 = + 8  ... whoops!!!      WRONG!

For this reason when I'm teaching this topic I try to keep away from the word minus altogether.  But, if my pupils use it I wouldn't correct them if it 'works' for them because I think it would confuse them.

http://www.mathsisfun.com/positive-nega … egers.html

Bob

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

Nehushtan

Member

Registered: 2013-03-09

Posts: 957

Re: parenthesis question

Parentheses are sometimes used to make an expression easier to read or interpret; they can be omitted if the expression remains unambiguous without them. For these

"-2 + -5 = -7" and "-2 x -5 = 10"

I would say the second one is fine, but I would put parentheses round the −5 in the first one, just so that the “+ −5” won’t be misinterpreted as “±5”.

When parentheses have an important role to play, they must not be omitted. For example:

This expression is equal to 3 × 3 = 9. If you omit the parentheses you have

which is a different expression altogether, evaluating to 1 + 6 = 7 instead. This is a case in which parentheses are important.

Last edited by Nehushtan (2014-01-13 21:19:29)

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phrontister

Real Member

From: The Land of Tomorrow

Registered: 2009-07-12

Posts: 4,818

Re: parenthesis question

Hi Bob,

-2 - (-5) = -7

(negative two) subtract (negative five).

If you leave out the brackets

-2 - -5 = -7

Shouldn't both answers be 3?

I stored "-2" and "-5" into separate memory locations (A and B respectively) on my calculator, and B - A = 3.

Most online evaluation calculators into which I fed "-2--5" went cross-eyed.

Image is from MIF's page you gave the link to.

Last edited by phrontister (2014-01-13 22:55:48)

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