=(19/20)int xdx/(x^2+4)-(4/5)int dx/(x^2+4)-(19/20)int dx/(x+4)=
=(19/40)ln(x^2+4) - (2/5)arctg (x/2) - (19/20)ln(x+4)+С
Это верно.

x^4-81=(x -3)(x+3)(x^2+9)