E(x)=(x+3)/(x+4)=(x+3+1-1)/(x+4)=(x+4)/(x+4)-1/(x+4)=1-1/(x+4)
2E(a)=2-2/(a+4)
2E(a)∈Z=>2∈Z si -2/(a+4)∈Z
-2/(a+4)∈Z=>(-a-4)|2=>(-a-4)∈D₂=>(-a-4)∈{-2,-1,1,2}|+4=>-a∈{2,3,5,6}|(-1)=>a∈{-6,-5,-3,-2}
