Tìm x,biết: a) 6x(3 - x) + 6(x^2)- 15x = 18

Hướng dẫn giải

a) \(6x(3 - x) + 6{x^2} - 15x = 18\)

\(18x - 6{x^2} + 6{x^2} - 15x = 18\)

\(3x = 18\)

\(x = 6\)

Vậy \(x = 6.\)

b) \(4\left( {x + 3} \right)\left( {{x^2} - 3x + 9} \right) - x\left( {4{x^2} - 3x} \right) = 6x + 108\)

\(4\left( {{x^3} + {3^3}} \right) - \left( {4{x^3} - 3{x^2}} \right) = 6x + 108\)

\(4{x^3} + 108 - 4{x^3} + 3{x^2} - 6x - 108 = 0\)

\(3{x^2} - 6x = 0\)
\(3x\left( {x - 2} \right) = 0\)

\(3x = 0\) hoặc \(x - 2 = 0\)

\(x = 0\) hoặc \(x = 2.\)

Vậy \(x \in \left\{ {0;\,\,2} \right\}.\)