# -7y = 0

• Mar 3rd 2006, 06:23 AM
Euclid Alexandria
-7y = 0
I came out with the right answer, but did I work this out correctly?

-7y = 0
-7y/-7 = 0/-7
-y = 0
-y/-y = 0/-y
y=0
• Mar 3rd 2006, 07:45 AM
earboth
Quote:

Originally Posted by Euclid Alexandria
I came out with the right answer, but did I work this out correctly?

-7y = 0
-7y/-7 = 0/-7
-y = 0
-y/-y = 0/-y
y=0

Hello,

you have made only a minor mistake:
-7y/-7 = 0/-7
y = 0

because -7y/-7 = +y the last two lines aren't necessary (and slightly wrong).

Greetings

EB
• Mar 3rd 2006, 08:23 AM
also going from
\$\displaystyle -y = 0\$
and you then said
\$\displaystyle -y/-y = 0/-y\$
this is not equivalent to:
\$\displaystyle y=0\$
as you said.

so what you want to do here to get the solution is divide both sides by \$\displaystyle -1\$
\$\displaystyle -y/-1 = 0/-1\$
\$\displaystyle y=0\$

since it turns out \$\displaystyle y=0\$
before you were trying to say \$\displaystyle -y/-y\$ which is \$\displaystyle 0/0\$ which is undefined.
• Mar 4th 2006, 07:45 AM
Euclid Alexandria
Thanks, guys. I couldn't remember the rule for getting a negative variable to be positive. You always divide both sides by -1?
• Mar 4th 2006, 05:42 PM
you could either divide by -1, multiply by -1, the effect is the same. you don't tend to think of it as "multiplying/dividing by -1" though. you just change the sign of both sides. for instance if
\$\displaystyle -x = -4\$
then
\$\displaystyle x = 4\$
you are simply changing the sign of both sides. you don't really need to explain yourself when doing that :)
\$\displaystyle -y=17\$
then changing the signs of each side gives:
\$\displaystyle y=-17\$
however multiplying through by -1 is a more common way of thinking about it than dividing through by -1.
• Mar 5th 2006, 03:50 AM
topsquark
Quote: