(a) Tìm x , biết: − 7/3 x = − 21/18 ; (b) Thực hiện phép nhân: ( x^2 − 8 ) ( x^3 + 2 x + 4 ) .

a) \(\frac{{ - 7}}{{3x}} = \frac{{ - 21}}{{18}}\)

\(3x = \frac{{ - 7.18}}{{ - 21}}\)

\(3x = - 6\)

\(x = - 2\)

Vậy \(x = - 2\).

b) \(\left( {{x^2} - 8} \right)\left( {{x^3} + 2x + 4} \right)\)

\( = {x^2}.\left( {{x^3} + 2x + 4} \right) - 8.\left( {{x^3} + 2x + 4} \right)\)

\( = {x^2}.{x^3} + {x^2}.2x + {x^2}.4 - 8.{x^3} - 8.2x - 8.4\)

\( = {x^5} + 2{x^3} + 4{x^2} - 8{x^3} - 16x - 32\)

\( = {x^5} - 6{x^3} + 4{x^2} - 16x - 32\).