Сравните:
1 1/4⋅4/7+2 1/3⋅4/7 и (1 1/4+2 1/3)⋅4/7

(1 1/4*4/7+2 1/3*4/7) - ((1 1/4+2 1/3)*4/7) = 1 1/4*4/7+2 1/3*4/7 - 1 1/4*4/7 - 2 1/3*4/7 (тут мы сокращаем:1 1/4*4/7 и -1 1/4*4/7; 2 1/3*4/7 и -2 1/3*4/7) = 0⇒1 1/4*4/7+2 1/3*4/7 = (1 1/4+2 1/3)*4/7