## Tìm x biết : a. (5x-4)^2 – 49x^2 = 0 b. x^2 +3x-10 =0

Question

Tìm x biết :
a. (5x-4)^2 – 49x^2 = 0
b. x^2 +3x-10 =0

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8 tháng 2020-10-15T12:12:10+00:00 2 Answers 86 views 0

1. a, (5x-4)^2-49x^2=0

⇔(5x-4)^2-(7x)^2=0

⇔(5x-4-7x)(5x-4+7x)=0

⇔(-2x-4)(12x-4)=0

$$⇔\left[ \begin{array}{l}-2x-4=0\\12x-4=0\end{array} \right.⇔\left[ \begin{array}{l}x=-2\\x=\dfrac13\end{array} \right.$$

b, x^2+3x-10=0

⇔x^2-2x+5x-10=0

⇔x(x-2)+5(x-2)=0

⇔(x+5)(x-2)=0

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

2. Đáp án:

a)$$\left[ \begin{array}{l}x=-2\\x=\dfrac{1}{3}\end{array} \right.$$

b)$$\left[ \begin{array}{l}x=2\\x=-5\end{array} \right.$$

Giải thích các bước giải:

$a)(5x-4)^2-49x^2=0$

$⇔(5x-4-7x).(5x-4+7x)=0$

$⇔(-2x-4).(12x-4)=0$

⇔$$\left[ \begin{array}{l}-2x-4=0\\12x-4=0\end{array} \right.$$

⇔$$\left[ \begin{array}{l}x=-2\\x=\dfrac{1}{3}\end{array} \right.$$

$b)x^2+3x-10=0$

$⇔x^2-2x+5x-10=0$

$⇔x.(x-2)+5.(x-2)=0$

$⇔(x-2).(x+5)=0$

$$\left[ \begin{array}{l}x-2=0\\x+5=0\end{array} \right.$$

$$\left[ \begin{array}{l}x=2\\x=-5\end{array} \right.$$