Toán Lớp 8: CÂU 2 : Tìm x biết: (4đ) a) 5x(x+2) – 5x (mũ 2 ) + 40 = 0 b) x 2 + 2x + 1 = 81 c) x 2 – 6x + 9 = 7

Question

Toán Lớp 8:  CÂU 2 : Tìm x biết: (4đ) a) 5x(x+2) – 5x (mũ 2 ) + 40 = 0 b) x 2 + 2x + 1 = 81 c) x 2 – 6x + 9 = 7

tonhu12 · Answer

a, 5x(x+2)-5x^2+40=0   &hArr; 5x^2+10x-5x^2=-40   &hArr;10x=-40   &hArr;x=-40:10   &hArr;x=-4   b, x^2+2x+1=81   &hArr;(x+1)^2=81   &hArr;(x+1)^2=9^2   &hArr;x+1=9  hoặc  &hArr;x+1=-9   &hArr;x=8              &hArr;x=-10   c,  x^2-6x+9 =7    &hArr;(x-3)^2=7   &hArr;x-3= $ sqrt{7}$  hoặc  &hArr;x-3=-$ sqrt{7}$   &hArr;x= $ sqrt{7}$+3             &hArr;x= -$ sqrt{7}$+3

ngoclanquynh · Answer

#kehuydiet   a/ $5x(x+2)-5x^2+40=0$   $&hArr; 5x^2+10x-5x^2+40=0$   $&hArr; 10x+40=0$   $&hArr; x=-4$   b/ $x^2+2x+1 = 81$   $&hArr; (x+1)^2=81$   $&hArr; x+1=9$ hoặc $x+1=-9$   $&hArr; x=8$ hoặc $x=-10$   c/ $x^2-6x+9=7$   $&hArr; (x-3)^2=7$   $&hArr; x-3 =  sqrt 7$ hoặc $x-3 = - sqrt 7$   $&hArr; x= sqrt 7 +3$ hoặc $x=- sqrt 7 +3$