Question

Cho B= 4x² -5x -1/5 . Tính B khi \(|\)x\(|\) = 3/2

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Answer 1

|x|=3/2

=>\(\left[{}\begin{matrix}x=\dfrac{3}{2}\\x=-\dfrac{3}{2}\end{matrix}\right.\)

Khi x=3/2 thì \(B=4\cdot\left(\dfrac{3}{2}\right)^2-5\cdot\dfrac{3}{2}-\dfrac{1}{5}=4\cdot\dfrac{9}{4}-\dfrac{15}{2}-\dfrac{1}{5}=\dfrac{13}{10}\)

Khi x=-3/2 thì \(B=4\cdot\left(-\dfrac{3}{2}\right)^2-5\cdot\dfrac{-3}{2}-\dfrac{1}{5}\)

\(=4\cdot\dfrac{9}{4}+5\cdot\dfrac{3}{2}-\dfrac{1}{5}\)

\(=9+\dfrac{15}{2}-\dfrac{1}{5}=\dfrac{163}{10}\)