б) 1 5c-d - 1 5c+d;

1/(5c-d) - 1/(5c+d) = (5c+d)/((5c-d)(5c+d)) - (5c-d)/((5c+d)(5c-d)) = (5c+d - (5c-d))/((5c)²-d²) = (5c+d-5c+d)/(25c²-d²) = (2d)/(25c²-d²).

Ответ: (2d)/(25c²-d²)