kitchen table math, the sequel: help desk - variables and expressions

## Friday, July 1, 2011

### help desk - variables and expressions

from Glencoe Algebra 1 Skills Practice (pdf file):
the difference of 17 and 5 times a number
How do you write this?

17 - 5n

I don't think I've seen a number problem worded that way.

PWN the SAT said...

I'd much rather it said "difference between."

Anne Dwyer said...

You have written it correctly. I tell my students that as soon as they see the word 'difference', they should write parenthesis with a subtraction in the middle. Then put the first number first and the second number (expression in this case) second:

(17-5n)

You may not need the parenthesis, but it doesn't hurt. You may need the parenthesis depending on the wording of the test of the problem. For example, it is says, "The difference of 17 and 5 times a number divided by 4 is n", then the parenthesis would be required because you would divide the entire expression by 4.

Richard I said...

Is there not an assumption that an absolute value expression is required here? Since we don't know the value of n, we can't be sure that 5n is less than 17.

Is the difference of 4 and 6 the same as the difference of 6 and 4? I always assumed they were.

Eowyn said...

That's just a horrible way of expressing 17-5n. The english, she is being abused.

SteveH said...

Richard is right. Difference means absolute value. There is no assumption that the first number is greater than the second number. I can't say that I ever heard of a negative difference.

If you use absolute value, you don't have to worry about which term you subtract from the other. It's the distance between the two numbers on the number line.

Linda Seebach said...

The English, she is saying, "the difference of 17 and 5 times a number" is 12 times a number, n'est-ce pas?

Catherine Johnson said...

Difference means absolute value.

That's what I thought when I read the question; I always think of the 'difference' between two numbers as the 'distance' between them, which is an absolute value.

(right?)

I'll have to notice, though, how these questions are normally written.

It may be that I was thrown by the use of 'of' instead of 'between.'

Catherine Johnson said...

The english, she is being abused.

lollllll----

ditto that