Toán Lớp 8: bài 3:Tìm x biết : a)3x^2-15x=0 b)x.(x-4)+x-4=0 c)x^2+11x+30=0 d)x^4-6x^2+7x-6=0

Question

Toán Lớp 8:  bài 3:Tìm x biết : a)3x^2-15x=0 b)x.(x-4)+x-4=0 c)x^2+11x+30=0 d)x^4-6x^2+7x-6=0

ngoclandiep · Answer

a)   3x^2 - 15x = 0   &hArr; 3x(x - 5) = 0   &rArr; x(x - 5) = 0   &rArr;  ( left[  begin{array}{l}x = 0  x = 5 end{array}  right. )    b)   x(x - 4) + (x - 4) = 0   &hArr; (x - 4)(x + 1) = 0   &rArr;  ( left[  begin{array}{l}x = 4  x = -1 end{array}  right. )    c)   x^2 + 11x + 30 = 0   &hArr; x^2 + 5x + 6x + 30 = 0   &rArr; x(x + 5) + 6(x + 5) = 0   &rArr; (x + 5)(x + 6) = 0   &rArr;  ( left[  begin{array}{l}x = -5  x = -6 end{array}  right. )    d)   x^4 - 6x^2 + 7x - 6 = 0   &hArr; x^4 - 2x^3 + 2x^3 - 4x^2 - 2x^2 + 4x + 3x - 6 = 0   &hArr; x^3(x - 2) + 2x^2(x - 2) - 2x(x - 2) + 3(x - 2) = 0   &hArr; (x - 2)(x^3 + 2x^2 - 2x + 3) = 0   &rArr; x - 2 = 0 (do  x^3 + 2x^2 - 2x + 3 > 0)   &rArr; x = 2