`y'=3x^2+4mx=0<=>[(x=0),(x=-4/3m):}` `(m ne 0)`
`=>[(y=-m),(y=32/27 m^3-m):}`
`=>A(0;-m),B(-4/3m;32/27 m^3-m)`
Để `\triangle OAB` vuong tại `O`
`=>\vec{OA}.\vec{OB}=0`
`<=>(0;-m).(-4/3m;32/27 m^3 -m)=0`
`<=>0.(-4/3m)-m(32/27 m^3-m)=0`
`<=>m^2(32/27m^2 -1)=0`
`<=>[(m=0(L)),(m=+-[3\sqrt{6}]/8 (t//m)):}`
Vậy `m=+-[3\sqrt{6}]/8`.