Question

Cho a+b=1. Tính giá trị biểu thức A= 2(a³+b³)-3(a²+b²)

Answer 1

\(A=2\left(a^3+b^3\right)-3\left(a^2+b^2\right)\)

\(A=2\left(a+b\right)\left(a^2-ab+b^2\right)-3\left(a^2+b^2\right)\)

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

Thay a + b = 1

\(A=2\left(1-3ab\right)-3\left(1-2ab\right)\)

\(A=2-6ab-3+6ab\)

\(A=-1\)