#### nikkimckelvy

##### New member

- Joined
- May 18, 2008

- Messages
- 1

I need a website that explains the logic behind the different formulas. Without that, I'm lost. I need to know when to use the formulas and why to use them. The knowledge of why they were invented and the logic motivating the people who discovered it would also be helpful. If I can understand that, I can usually understand when to apply the problem.

For example, if x^2-y^2= 28 and x-y=8 what is the average of x and y? If I apply FOIL then 28= (x-y) (x+y). That means that 28=8(x+y). From here what do I do? Do I do 28=8x+8y or do I do 28/8=x+y? That would mean that 7/2=x+y, and none of those answers the question of what is the average of x and y.

I got that question from Barrons GRE. Understanding the concepts behind this stuff would really help. So would understanding when this is applicable in life. Any help would be appreciated.

Sincerely,

Nikki