elementary help
- Thread starter jimbir
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A minus sign can be changed to a plus sign with a multiplication by minus 1, that is
a - b = a + (-1) * b
and
- b = + (-1) * b = (-1) * b
Now if you have two minus signs together we have, going the long way around,
a - (-b) = a + (-1) * (-b) = a + (-1) * (-1) * b = a + (1) * b = a + b
Or, we can go the short way around and just remember that when we have two minus signs in a row like that, we can just change them both to plus signs.
jimbir
New member
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- Apr 11, 2015
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let me take the long way around in a different direction. As I understand it, anything you do to one side of an equation, you must do the same to the other. Therefore, taking my original equation,
7-(-3)=10 and multiplying both sides by -1, I get:
-7+(-3)= -10 Multiplying by -1 again, I get:
7+(+3)=10 = 7+3=10
Is my train of thought correct?
Jim
7-(-3)=10 and multiplying both sides by -1, I get:
-7+(-3)= -10 Multiplying by -1 again, I get:
7+(+3)=10 = 7+3=10
Is my train of thought correct?
Jim
Steven G
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- Dec 30, 2014
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- 14,586
That works. I tend to think of it like this: 7-(-3) is NOT 7-3 so it must be 7+3.
You asked for the logic behind it so here goes. Suppose you have $100 and you add bills to your table (where you keep your bills). Suppose your bills are $20 and $30. Remember you ADDED the bills to the table. So you have $100 + (-20) + (-30) which is 100-20-30 or $50. Now suppose the person you owe the $20 says to forget about paying her. So you are now removing that bill, ie you are subtracting that bill. The - (-30) becomes +30. I hope this helps.
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