Question

A = 64a^3 - 8b^3 , biết 4a - 2b = -2 ; ab = 5

Answer 1

A = 64a³ - 8b³

= (4a)³ - (2b)³

= (4a - 2b)(16a² - 8ab + 4b²)

= (4a - 2b)(16a² - 16ab + 4b² + 8ab)

= (4a - 2b)[(4a - 2b)² + 8ab]

= (-2).[(-2)² + 8.5]

= (-2).(4 + 40)

= (-2).44

= -88

Answer 2

\(A=64a^3-8b^3=\left(4a\right)^3-\left(2b\right)^3=\left(4a-2b\right)\left(16a^2-8ab+4b^2\right)\)

Ta có: \(\left(4a-2b\right)^2=16a^2-16ab+4b^2=\left(-2\right)^2=4\)

\(\Leftrightarrow16a^2+4b^2=4+16ab=4+16.5=84\)

\(\Rightarrow A=\left(4a-2b\right)\left(16a^2-8ab+4b^2\right)\)

\(=-5\left(84-8.5\right)=44.-5=-220\)