Bài 42 trang 15 SBT toán 7 tập 1

Đề bài

Tìm \(x ∈\mathbb Q\), biết rằng:

\({\rm{a}})\;{\left( {x - \displaystyle {1 \over 2}} \right)^2} = 0\)

\(b)\;{\left( {x - 2} \right)^2} = 1\)

\(c)\;{\left( {2{\rm{x}} - 1} \right)^3} =  - 8\)

\({\rm{d}})\;{\left( {x + \displaystyle {1 \over 2}} \right)^2} = \displaystyle {1 \over {16}}\)

Lời giải

\({\rm{a}})\;{\left( {x - \displaystyle {1 \over 2}} \right)^2} = 0 \)

\(\Rightarrow x - \displaystyle {1 \over 2} = 0 \)

\(\Rightarrow x = \displaystyle {1 \over 2}\) 

\(b)\;{\left( {x - 2} \right)^2} = 1 \)

\( \Rightarrow \left( {x - 2} \right) = {1^2}\)

\(\Rightarrow \left[ \matrix{
x - 2 = 1 \hfill \cr 
x - 2 = - 1 \hfill \cr} \right.\)

\(\Rightarrow \left[ \matrix{
x = 3 \hfill \cr 
x = 1 \hfill \cr} \right.\)

\(c)\;{\left( {2{\rm{x}} - 1} \right)^3} =  - 8\)

\(\Rightarrow {\left( {2{\rm{x}} - 1} \right)^3} = {\left( -2 \right)^3}\)

\(\Rightarrow 2{\rm{x}} - 1 =  - 2\)

\( \Rightarrow 2x =  - 2 + 1\)

\( \Rightarrow 2x =  - 1\)

\(\Rightarrow x = \displaystyle  - {1 \over 2}\)

\(\displaystyle {\rm{d)}}\;{\left( {x + {1 \over 2}} \right)^2} = {1 \over {16}}\)

\(\displaystyle \Rightarrow {\left( {x + {1 \over 2}} \right)^2} = {\left( {{1 \over 4}} \right)^2} \)

\( \Rightarrow \left[ \begin{array}{l}x + \dfrac{1}{2} = \dfrac{1}{4}\\x + \dfrac{1}{2} = - \dfrac{1}{4}\end{array} \right.\)

\( \Rightarrow \left[ \begin{array}{l}x = \dfrac{1}{4} - \dfrac{1}{2}\\x = - \dfrac{1}{4} - \dfrac{1}{2}\end{array} \right.\)

\( \Rightarrow \left[ \begin{array}{l}x = - \dfrac{1}{4}\\x = - \dfrac{3}{4}\end{array} \right.\)


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