Объяснение:

А)

(2х + у)/6 - (у - 4х)/6 = (2х+у - (у-4х))/6 =

(2х + у - у + 4х)/6 = 6х/6 = х

Б)

(m + 3)/5m + (8 - 3t)/20t =

4t(m + 3)/(4t * 5m) + m(8 - 3t)/(m*20t) =

(4mt + 12t)/20mt + (8m - 3mt)/20mt =

(4mt + 12t + 8m - 3mt)/20mt =

(mt + 12t + 8m)/20mt

В)

20/(S² + 45) - 5/S=

20S - 5(s² + 45) / S*(S² + 45) =

20s - 5s² - 225 / s³ + 45S =

-5s² + 20s - 225 / s³ + 45s