Verified answer

1) (x + y) + (x - y) = 2x

(x + y) - (x - y) =

= x + y - x + у = 2у

2) (а² - а + 4) + (-а² - а - 4) =

= а² - а + 4 - а² - а - 4 = -2а

а² - а + 4 - (-а² - а - 4) =

= а² - а + 4 + а² + а + 4 = 2а² + 8

3) (b³ - 8) + (-b³ - 8) =

= b³ - 8 -b³ - 8 = -16

b³ - 8 - (-b³ - 8) =

= b³ - 8 + b³ + 8 = 2b³