Resposta:

k)

(2/3m²-1/2m+1)+(-1/2m²+2/3m-1)=

2/3m²-1/2m+1-1/2m²+2/3m-1=

1/6m² +1/6m

l)

(1/2m-1/3mn+2)+(mn+1-m/2)=

1/2m-1/3mn+2+mn+1-m/2=

2/3mn +1

m))

(2ab²-1/3a²b)-3/4ab²)-3/4ab²-a²b)=

2ab²-1/3a²b-3/4ab²-3/4ab²-a²b=

-4/3a²b +1/2a²b