# Subtracting Integers - sea level difference

• February 12th 2006, 05:13 PM
Euclid Alexandria
Subtracting Integers - sea level difference
From the book: "If the elevation of Lake Superior is 600 feet above sea level and the elevation of the Caspian Sea is 92 feet below sea level, find the difference of the elevations."

My answer: 600 - (-92) = 600 + 92 = 692 ft

Am I correct? This seems very obvious to me, but I'm just double checking.
• February 12th 2006, 08:01 PM
earboth
Quote:

Originally Posted by Euclid Alexandria
My answer: 600 - (-92) = 600 + 92 = 692 ft

Am I correct? This seems very obvious to me, but I'm just double checking.

Hello,

you've got it. It's perfect: Congratulations!

Bye
• February 13th 2006, 05:57 PM
Euclid Alexandria
Thanks!
• June 10th 2006, 04:44 PM
Waveform
Hi I’m kind of new to this stuff, I never had algebra before.
Can I just ask a quick question?

If you know lake-A is 600+Feet above see level and another lake is -92 feet, why doesn’t algebra just say 600 + 92 = the difference?

What I mean is: 600 – (-92)
Is the same as saying 600 + 92.

So I guess what I’m asking is, what is the advantage or better yet how dose it relate in the real world? Sorry for the amateur question but I’m just trying to keep my thinking in line. Some of these sound too easy which is what freaks me out, because I know something crazy is right around the corner and I know that all these principles will matter once I start getting into this stuff. Can someone please explain why Algebra problems are twisted or worded different, all for the same answer? What would be a real would example for this?

• June 10th 2006, 10:16 PM
CaptainBlack
Quote:

Originally Posted by Waveform
Hi I’m kind of new to this stuff, I never had algebra before.
Can I just ask a quick question?

If you know lake-A is 600+Feet above see level and another lake is -92 feet, why doesn’t algebra just say 600 + 92 = the difference?

What I mean is: 600 – (-92)
Is the same as saying 600 + 92.

The problem is intended to show that subtracting a negative number is the
same as adding the corresponding positive number. So it is obvious to you
that the difference in heights is 600+92, and this illustrates that 600-(-92)
is the same thing.

At some point you will see the general rule

$x-(-y)=x+y$

and this problem just shows why we want the rule that minus times minus
as used here is plus.

RonL
• July 11th 2006, 05:35 PM
Quick
I realize that this is really late.
I would like to point out why in algebra you say $600-(-92)$
it is because in moderately low to highly advanced math you are NOT allowed to use logic, only formulas. Therefore, I would say the formula for solving this problem is $a-b=c$ and then I would substitute the values into this equation $(600)-(-92)=692$

Another reason: when you start to get into bigger equations, you will have to use the cumulative property of addition and other properties...
an example:
$2x^2-3x-x^2+5x\quad\Rightarrow\quad2x^2+(-3x)+$ $(-x^2)+5x\quad\Rightarrow\quad2x^2+$ $(-x^2)+5x+(-3x)\quad\Rightarrow\quad x^2+2x$