3/4+(-3/10)= -12 3/8+(4 11/2) -2 5/9 +(-2 5/8)= -6 7/12+7 4/9= за ранее !

-3/4 + (-3/10) = -15/20 + (- 6/20) = - 21/20= - 1  1/20 
-2  5/9 + (- 2 5/8 ) = -2  40/70  + (- 2  45/72)  = - 5  13/72
-6  7/12 + 7 4/9 =  7  16/36 - 6  21/36 =  31/36

- 12  3/8 +  4  11/2 = - 12 3/8   +  9 1/2  =
 =-12 3/8 + 9  4/8 = -2  7/8
или 
-12 3/8  + 41  1/2 = 41 4/8  -12  3/8  = 29  1/8 

-3/4+(-3/10) = -(3/4+3/10) = =-(15/20+6/20) = =-21/20=-1целая 1/20

-2целых 5/9+(-2ц.5/8) = -(2 ц.5/9+2ц.5/8) = -(4ц.+(40/72+45/72)) = -4ц. 85/72 = -5ц.13/72

-6ц.7/12+7ц.4/9 = 7ц.4/9-6ц.7/12 = 7ц.16/36 -6ц.21/36 = 31/36