№760
1) (a-b)/ab - (a-c)/ac = (ac - bc - ab + bc)/abc = (ac - ab)/abc =
=a(c-b)/abc = (c-b)/bc
2) = (b² + a + b)/ a²b²
остальное не видно, извините
№ 762
1) a/(a²-1) - 1/(1-a²) = a/(a²-1) + 1/(a²-1) = (a+1) / (a²-1) = 1/(a-1)
2) = 3y/(4x² - 9y²) - 2x/(4x² - 9y²) = (3y-2x)/(4x² - 9y²) = -(2x-3y)/(4x²-9y²) =
= -1 / (2x + 3y)