Question

Bài 1 Cho \(|x|\)= 20 ; \(|y|\)=9 . Tính x-y biết

a) x>0, y<0

b) x.y >0

Answer 1

Giải:

a) \(\left|x\right|=20\Leftrightarrow x=\left[{}\begin{matrix}20\\-20\end{matrix}\right.\)

Mà \(x>0\)

\(x=20\)

\(\left|y\right|=9\Leftrightarrow x=\left[{}\begin{matrix}9\\-9\end{matrix}\right.\)

Mà \(y< 0\)

\(y=-9\)

\(\Leftrightarrow x-y=20-\left(-9\right)=20+9=29\)

Vậy ...

b) \(x.y>0\)

⇒ x và y cùng dấu

\(\Leftrightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}x=20\\y=9\end{matrix}\right.\\\left\{{}\begin{matrix}x=-20\\y=-9\end{matrix}\right.\end{matrix}\right.\)

* \(\left\{{}\begin{matrix}x=20\\y=9\end{matrix}\right.\)

\(\Leftrightarrow x-y=20-9=11\)

* \(\left\{{}\begin{matrix}x=-20\\y=-9\end{matrix}\right.\)

\(\Leftrightarrow x-y=-20-\left(-9\right)=-20+9=-11\)

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