Ta có:
\(A=5x^2-\left[4x^2-3x\left(x-20\right)\right]\)
\(\Leftrightarrow A=5x^2-\left[4x^2-\left(3x^2-60x\right)\right]\)
\(\Leftrightarrow A=5x^2-\left(4x^2-3x^2+60x\right)\)
\(\Leftrightarrow A=5x^2-\left(x^2+60x\right)\)
\(\Leftrightarrow A=5x^2-x^2-60x\)
\(\Leftrightarrow A=4x^2-60x\)
Thay \(x=-\dfrac{1}{2}\) vào biểu thức \(A=4x^2-60x\), ta được:
\(A=4\cdot\left(-\dfrac{1}{2}\right)^2-60\cdot\dfrac{-1}{2}=4\cdot\dfrac{1}{4}+30=31\)
\(A=5x^2-\left[4x^2-3x\left(x-20\right)\right]=5x^2-\left(4x^2-3x^2+60x\right)=5x^2-4x^2+3x^2-60x=4x^2-60x\)Với \(x=\dfrac{-1}{2}\Rightarrow A=4.\left(\dfrac{-1}{2}\right)^2-60.\left(\dfrac{-1}{2}\right)=31\)
