ta có : a-b=6
nên (a-b)^2=36
a^2-2ab+b^2=36
mà ab=9
=>a^2+b^2-2.9=36
a^2+b^2=54
lại có A=a^3-b^3-84
=(a-b)(a^2+ab+b^2)-84
=6.(a^2+b^2+9)-84
=6.(54+9)-84
=378-84=294
Ta có : \(a-b=6\Rightarrow\left(a-b\right)^2=36\Rightarrow a^2-2ab+b^2=36\Rightarrow a^2+b^2=54\)
\(A=a^3-b^3-84=\left(a-b\right)\left(a^2+b^2+ab\right)-84=6.\left(54+9\right)-84=294\)
Ta có : \(a-b=6\)
\(\Leftrightarrow\left(a-b\right)^2=36\)
\(\Leftrightarrow a^2-2ab+b^2=36\)
\(\Leftrightarrow a^2-2.9+b^2=26\)
\(\Leftrightarrow a^2+b^2-18=36\)
\(\Leftrightarrow a^2+b^2=54\)
Lại có : \(A=a^3-b^3-84\)
\(\Leftrightarrow A=\left(a-b\right)\left(a^2+ab+b^2\right)-84\)
\(\Leftrightarrow A=6\left(54+9\right)-84\)
\(\Leftrightarrow A=6.63-84\)
\(\Leftrightarrow A=294\)
Vậy \(A=294\)
