If a + b = 2, a + c = 7, and b + c = 9, find a + b + c.
there's 2 ways to solve this.
1. you can just experiment, and you can see that a=0, b=2, and c=7
2. we see that a+b+a+c=b+c
from there, you can find the other values :)
Add all three equations together to get 2a+2b+2c=18. We divide this equation by 2 to get a+b+c=9.
2+7+9=a+b+a+c+b+c=2a=2b+2c so we then finally divide by 2 to get a+b+c=9