Toán Lớp 8: Tìm x: a) 3x(x+1)+5x=0 b) x mũ 2-5x=0 c) 4x mũ 2 – 1-( 2x+1 )=0 d) x mũ 2-7x+10=0

0 bình luận về “Toán Lớp 8: Tìm x: a) 3x(x+1)+5x=0 b) x mũ 2-5x=0 c) 4x mũ 2 – 1-( 2x+1 )=0 d) x mũ 2-7x+10=0”

1. Lời giải:

   a)

   3x(x+1)+5x=0

   ⇔3x^(2)+3x+5x=0

   ⇔3x^(2)+8x=0

   ⇔x(3x+8)=0

   $⇔\left[\begin{matrix}x=0\\ 3x+8=0\end{matrix}\right.$

   $⇔\left[\begin{matrix}x=0\\ 3x=-8\end{matrix}\right.$

   $⇔\left[\begin{matrix}x=0\\ x=\dfrac{-8}{3}\end{matrix}\right.$

   Vậy x=0 hoặc x=-8/3

   b)

   x^(2)-5x=0

   ⇔x(x-5)=0

   $⇔\left[\begin{matrix}x=0\\ x-5=0\end{matrix}\right.$

   $⇔\left[\begin{matrix}x=0\\ x=5\end{matrix}\right.$

   Vậy x=0 hoặc x=5

   c)

   4x^(2)-1-(2x+1)=0

   ⇔(2x)^(2)-1-(2x+1)=0

   ⇔(2x-1)(2x+1)-(2x+1)=0

   ⇔(2x+1)(2x-1-1)=0

   ⇔(2x+1)(2x-2)=0

   ⇔2(2x+1)(x-1)=0

   ⇔(2x+1)(x-1)=0

   $⇔\left[\begin{matrix}2x+1=0\\ x-1=0\end{matrix}\right.$

   $⇔\left[\begin{matrix}2x=-1\\ x=1\end{matrix}\right.$

   $⇔\left[\begin{matrix}x=\dfrac{-1}{2}\\ x=1\end{matrix}\right.$

   Vậy x=-1/2 hoặc x=1

   d)

   x^(2)-7x+10=0

   ⇔x^(2)-2x-5x+10=0

   ⇔x(x-2)-5(x-2)=0

   ⇔(x-2)(x-5)=0

   $⇔\left[\begin{matrix}x-2=0\\ x-5=0\end{matrix}\right.$

   $⇔\left[\begin{matrix}x=2\\ x=5\end{matrix}\right.$

   Vậy x=2 hoặc x=5

   Trả lời