1) у = 6√х + 3/х
у' = 6/2√х - 3/х² = 3/√х - 3/х²
2)у = (х² -2)(х² +4)
у'= (х²-2)' * (x² +4) + (x² -2) * (x² +4)' = 2x(x² +4) +2x(x² - 2) =
= 2x³ +4x +2x³ - 4x = 4x³
3) f(x) = (2x -5)/(5-x)
f'(x) = (2(5-x) +1*(2x -5))/(5 -x)² = (10 -2x +2x -5)/(5 -x)² = 5/(5-x)²