1. 5 5/9 + 4 7/9 = 10 1/3
1) (5+4)+(5/9+7/9)
2) 9+4/3 = 9+1 1/3
3) 9+1+1/3 = 10+1/3 = 10 1/3
2. 3 20/21 + 16/21 = 4 5/7
1) 83/21+16/21 = 33/7 = 4 5/7
3. 3 11/12 + 5 7/12 = 9 1/2
1) (3+5)+(11/12+7/12)
2) 8 + 3/2 = 8 + 1 1/2
3) 8 + 1 + 1/2 = 9+1/2 = 9 1/2
4. 8 10/13 + 1 11/13 = 10 8/13
1) (8+1)+(10/13+11/13)
2) 9 + 21/13
3) 9 + 1 8/13
4) 9 + 1 + 8/13
5) 10 + 8/13 = 10 8/13
монотонно убывает на интервале знакопостоянства производной: x∈(-∞;0) 
