1. question.

why cant you cancel the (-1)'s in this fraction? ...they just solved the problem.

2. If you work it out:

2(-1)-1(4)
(-1)^2

is equal to -6. However if we cancel the (-1) on top so it's just 2(1) and cancel (-1)^2 to become just (-1) we get:

2(1)-1(4)

(-1)

which is 2. The problem is you have that " -1(4) " bit at the end. If it were just:

2(-1)
(-1)^2

Then you could definitely cancel the (-1). However, the " -1(4) " part screws it up! Hope this helps.

3. You can cancel it ! Look :

$\displaystyle \frac{2(-1) - 1(4)}{(-1)^2}$

$\displaystyle \frac{-2 - 4}{(-1)^2}$

$\displaystyle \frac{-(2 + 4)}{(-1)^2}$

$\displaystyle \frac{(-1)(2 + 4)}{(-1)^2}$

$\displaystyle \frac{2 + 4}{-1}$

$\displaystyle \frac{-(2 + 4)}{1}$

$\displaystyle -(2 + 4)$

$\displaystyle -2 - 4$

$\displaystyle -6$

Does it make sense ?

4. Your math makes perfect sense. However, i believe mojojojo meant why can't you cancel the (-1)'s in the first step of the equation. Which you can not. Am i wrong?

5. Yes, you are right. One has to rearrange it and factorize it first before cancelling out the minus 1.