Question

cho a+b=2 tính m=4(a^3+b^3)-6(a^2+b^2)

Answer 1

\(M=4.\left(a^3+b^3\right)-6\left(a^2+b^2\right)\)

\(=4.\left(a+b\right)\left(a^2-ab+b^2\right)-6a^2-6b^2\)

Thay a + b = 1 vào M ta có:

\(M=4.1\left(a^2-ab+b^2\right)-6a^2-6b^2\)

\(=4\left(a^2-ab+b^2\right)-6a^2-6b^2\)

\(=4a^2-4ab+4b^2-6a^2-6b^2\)

\(=-2a^2-4ab-2b^2\)

\(=-\left(2a^2+4ab+2b^2\right)\)

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

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

Tiếp tục thay a + b = 1 vào M ta có:

\(M=-2.1^2=-2\)

P/s: Ko chắc!