Đề bài
Tìm x, biết:
a) \(2x(x - 3) - x(2x + 3) = 18\)
b) \(x(5{x^2} - 2) + 5x(1 - {x^2}) = {3^4}\)
Lời giải chi tiết
\(a){\mkern 1mu} {\mkern 1mu} 2x\left( {x - 3} \right) - x\left( {2x + 3} \right) = 18 \\\Leftrightarrow {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} 2{x^2} - 6x - 2{x^2} - 3x = 18\\ \Leftrightarrow - 9x = 18 \\\Leftrightarrow x = 18:\left( { - 9} \right) = - 2\)
Vậy \(x = -2\)
\(b){\mkern 1mu} {\mkern 1mu} x\left( {5{x^2} - 2} \right) + 5x\left( {1 - {x^2}} \right) = {3^4}\\ \Leftrightarrow {\mkern 1mu} 5{x^3} - 2x + 5x - 5{x^3} = 81\\ \Leftrightarrow {\mkern 1mu} 3x = 81\\ \Leftrightarrow x = 81:3 = 27\)
Vậy \(x = 27\)