\(A=2\left(a^3+b^3\right)-3\left(a^2+b^2\right)\)
\(A=2\left(a+b\right)\left(a^2-ab+b^2\right)-3\left(a^2+b^2\right)\)
\(A=2\left(a+b\right)\left[\left(a+b\right)^2-3ab\right]-3\left[\left(a+b\right)^2-2ab\right]\)
Thay a + b = 1
\(A=2\left(1-3ab\right)-3\left(1-2ab\right)\)
\(A=2-6ab-3+6ab\)
\(A=-1\)