3. Выполните действия: a) 5(3/4 - 4/8) + 7(1/2 : 2/7), б) (1/3 - 6/7) * (3/4 + 8/9).

a) 5((3/4) - (4/8)) + 7((1/2) ÷ (2/7)) = 5(3/4 - 1/2) + 7(1/2 * 7/2) = 5(1/4) + 7(7/4) = 5/4 + 49/4 = 54/4 = 13 1/2. б) ((1/3) - (6/7)) * ((3/4) + (8/9)) = (-11/21) * (59/36) = -649/756 = -0,858. Ответы: a) 13 1/2, б) -0,858.