(а/а-в - а+в/а) х а-в/2в =
(a^2/a(a-b) - (a+b)*(a-b)/a(a-b))* a-b/2b
(a^2-a^2+ab-ab+b^2/a(a-b))* a-b/2b
(b^2/a(a-b))*a-b/2b
b/2a