Toán Lớp 8: Tìm x, biết: a) 3x( x -1) +x -1 =0; b)2(x+3) –x^3 -3x =0; c) 4x^2 – 25- (2x -5) (2x +7); d) x^3+ 27+ (x+3) (x -9) = 0;

Question

Toán Lớp 8:  Tìm x, biết: a) 3x( x -1) +x -1 =0; b)2(x+3) –x^3 -3x =0; c) 4x^2 – 25- (2x -5) (2x +7); d) x^3+ 27+ (x+3) (x -9) = 0;

ankim · Answer

Lời giải và giải thích chi tiết:    a). 3x( x -1) + (x -1) = 0     &hArr; 3x( x -1) + (x -1) = 0     &hArr; (x -1)(3x + 1) = 0    &rArr; x - 1 = 0 hay 3x + 1 = 0   &rArr; x = 1 hay 3x = - 1   &rArr; x = 1 hay x = (-1)/3   Vậy S = {1; (-1)/3}     b). 2(x+3) &ndash; x&sup2; - 3x = 0     &hArr; 2(x + 3) - x(x + 3) = 0     &hArr; (x + 3)(2 - x) = 0     &rArr; x +3 = 0 hay 2 - x = 0     &rArr; x = - 3 hay x = 2     Vậy S = {- 3; 2}     c). 4x&sup2; &ndash; 25 - (2x -5) (2x +7) = 0     &hArr; [(2x)&sup2; &ndash; 5&sup2;] - (2x -5) (2x +7) = 0     &hArr; (2x - 5)(2x + 5) - (2x -5)(2x +7) = 0     &hArr; (2x - 5)[2x + 5 - (2x +7)] = 0     &hArr; (2x - 5)(2x + 5 - 2x - 7) = 0     &hArr; (2x - 5)(- 2) = 0     &hArr; - 2 (2x - 5) = 0     &rArr; 2x - 5 = 0     &rArr; 2x = 5     &rArr; x = 5/2     Vậy S = {5/2}     d). x&sup3;+ 27 + (x+3) (x -9) = 0     &hArr; x&sup3; + 3&sup3; + (x+3) (x -9) = 0     &hArr; (x + 3)(x&sup2; - 3x + 9) + (x+3) (x -9) = 0     &hArr; (x + 3)(x&sup2; - 3x + 9 + x - 9) = 0     &hArr; (x + 3)(x&sup2; - 2x)  = 0     &hArr; (x + 3). x(x - 2) = 0     &hArr; x(x + 3)(x - 2) = 0     &rArr; x = 0 hay x + 3  = 0  hay x - 2 = 0      &rArr;x = 0 hay x = - 3  = 0  hay x = 2      Vậy S = {0; - 3; 2}     Ch&uacute;c bạn học tốt