Question

Why has she putten these parentheses? And the first number (xe2) goes from positive to negative ((-2)e2) and then back to positive (4)?

Answer 1

`x=-2\hspace{5em}y=2 \\\\[-0.35em] ~\dotfill\\\\ \cfrac{x^2 - y^3}{x^3 + y^2}\implies \cfrac{(-2)^2 - (2)^3}{(-2)^3 + (2)^2}\implies \cfrac{(-2)(-2)~~ - ~~(2)(2)(2)}{(-2)(-2)(-2)~~ + ~~(2)(2)} \\\\\\ \cfrac{(+4)~~ - ~~(+8)}{(-8)~~ + ~~(+4)}\implies \cfrac{4-8}{-8 + 4}\implies \cfrac{-4}{-4}\implies \text{\LARGE 1}`

for the sake of clarity, is better to parenthesize all, since plugging in the value for the variable, only includes the variable, no any exponents or signs in front.

that said, let's recall that minus * minus = plus, and that plus * plus = plus, however minus * plus = minus.

so you can think of it this way, if you have a negative value multiplied an "even times", it'll positivize, if you multiply it an "odd times", it'll go back to negative, so is not like it remains negative, it changed a few times and in the end negativized back.