Finding intersections by adding 2 equations

• Feb 16th 2009, 04:31 PM
TaylorTeamEdward324
Finding intersections by adding 2 equations
For each of the following pairs of equations, find the point of intersection by adding the two equations together. Remember: you might need to change the coeffients and/or signs of the variables before adding.

Help!

x-y = -1
x+y= 9
• Feb 16th 2009, 04:34 PM
Mush
Quote:

Originally Posted by TaylorTeamEdward324
For each of the following pairs of equations, find the point of intersection by adding the two equations together. Remember: you might need to change the coeffients and/or signs of the variables before adding.

Help!

x-y = -1
x+y= 9

If you add two equations, you are their left hand sides together, and their right hand sides together.

So you'd get:

\$\displaystyle x-y + (x+y) = -1+9 \$

\$\displaystyle x-y+x+y = 8 \$

\$\displaystyle 2x = 8 \$

\$\displaystyle x = 4 \$

Then just put x = 4 into one of the original equations to get y!

\$\displaystyle x + y = 9 \$

\$\displaystyle 4+y = 9 \$

\$\displaystyle y = 9-4\$

\$\displaystyle y = 5 \$
• Feb 16th 2009, 04:40 PM
TaylorTeamEdward324
Can you break it down?
Okay? What I don't understand is how you got the -1 and why you have the x - y (x+y)... I don't understand what the reasoning is behind that. Could you explain that to me?

Thanks so much for helping me... I AM getting the same answer but not really understanding the way I'm getting it.. make sense? LOL

Thanks again!!!