Question

(7/8-5/6) + (8/9-7/9)

Answer 1

(7/8-5/6) + (8/9-7/9) = 

Resolvendo por partes:
7/8 - 5/6 = ---mmc(6,8) = 24
21/24 - 20/24 = 1/24

8/9 - 7/9 = 1/9

Então fica assim:
(7/8-5/6) + (8/9-7/9) = 
 1/24 + 1/9 = ----mmc(9,24) = 72
3/72 + 8/72 = 11/72