$\text{ta có : }a^2 +5b^2 = 6ab\\\to a^2+5b^2-6ab=0\\\to (a-b)(a-5b)=0$ \(\to\left[ \begin{array}{l}a=b\\a=5b\end{array} \right.\)
`+\text{với }a=b\text(,thì :`
`D=(a^3+b^3)/(3ab^2+2a^2b)=(a^3+a^3)/(3a^3+2a^3)=2/5`
`+\text{với }a=5b\text(,thì :`
`D=(a^3+b^3)/(3ab^2+2a^2b)=((5b)^3+b^3)/(3(5b).b^+2.(5b)^2.b)=(126b^3)/(50^3+10b^3)=126/60=21/10`