Toán Lớp 8: Tìm x: a)(x+3)(x^2-3x+9)-x(x^2-3)=18 b)4x^2-5=4

Question

Toán Lớp 8:  Tìm x: a)(x+3)(x^2-3x+9)-x(x^2-3)=18 b)4x^2-5=4

quynhmaithu · Answer

Giải đáp:   a)x=-3   b)x&isin;{3/2;-3/2}   Lời giải và giải thích chi tiết:   a)(x+3)(x&sup2;-3x+9)-x(x&sup2;-3)=18   &hArr;x&sup3;+27-x&sup3;+3x=18   &hArr;(x&sup3;-x&sup3;)+3x+27=18   &hArr;3x+27=18   &hArr;3x=18-27   &hArr;3x=-9   &hArr;x=(-9):3   &hArr;x=-3   Vậy x=-3   b)4x&sup2;-5=4   &hArr;4x&sup2;=4+5   &hArr;4x&sup2;=9   &hArr;(2x)&sup2;=3&sup2;   &hArr;$ left[ begin{matrix} 2x=3   2x=-3 end{matrix} right.$   &hArr;$ left[ begin{matrix} x= dfrac{3}{2}   x=- dfrac{3}{2} end{matrix} right.$   Vậy x&isin;{3/2;-3/2}

lanhuongthu · Answer

a)(x+3)(x^2-3x+9)-x(x^2-3)=18   &hArr;x^3-3x^2+9x+3x^2-9x+27-x^3+3x=18   &hArr;27+3x=18   &hArr;3x=18-27   &hArr;3x=-9   &hArr;x=-3   Vậy x=-3   b)4x^2-5=4   &hArr;4x^2=4+5   &hArr;4x^2=9   &hArr;(2x)^2=3^2   &hArr;2x=3; 2x=-3   &hArr;x=3/2; x=-3/2   Vậy x=3/2; -3/2