r^4-5r³+6r²+4r-8=r²(r²-5r+6)+4(r-2)=r²(r-2)(r-3)+4(r-2)=(r-2)[r²(r-3)+4]=(r-2)(r³-3r²+4)=(r-2)(r³-2r²-r²+4)=(r-2)[r²(r-2)-(r+2)(r-2)]=(r-2)(r-2)(r²-r-2)=(r-2)²(r+1)(r-2)=(r-2)³(r+1)
