Bonjour,
1) 1620/6156+4725/4275
1620/6156 = 810/3078 = 405/ 1539 = 135/ 513 = 48/171 = 16/57
4725/4275 = 945/855= 189/171 = 63/57
16/57 + 63/57 = 79/57
2) 1/3+1/9+1/27+1/81+1/273
1/273 : une erreur?
1/3+1/9+1/27+1/81 =
27/81 + 9/81+ 3/81+1/81 = 40/81
et 1/273