計算2.
設\(x\)為有理數,將\((x+1)(x-2)\)的小數第一位予以「四捨五入」後所得的整數為\(1+5x\) ,則\(x\)的值為多少?
[解答]
假設\(\displaystyle x=\frac{m}{5},m\in \mathbb{Z}\)
則原數為\(\displaystyle (\frac{m}{5}+1)(\frac{m}{5}-2)\),四捨五入後的數字為\(\displaystyle 1+m\)
可知\(\left\{
\begin{array}{LL}
m+\frac{1}{2}\leq \displaystyle (\frac{m}{5}+1)(\frac{m}{5}-2)<m+1 \\
or \\
m+1\leq \displaystyle (\frac{m}{5}+1)(\frac{m}{5}-2)<m+\frac{3}{2}
\end{array}
\right.
\)
若是\(\displaystyle m+\frac{1}{2}\leq \displaystyle (\frac{m}{5}+1)(\frac{m}{5}-2)<m+1\)
整理化簡後可得 \(\displaystyle 62.5\leq m^2-30m<75\)
\(\displaystyle m^2-30m=63,64,\cdots ,73,74\),利用同餘篩選並檢驗發現只有64符合題意
解得\(m=32\)或是\(m=-2\),對應到\(\displaystyle x=\frac{32}{5},x=\frac{-2}{5}\)
若是\(\displaystyle m+1\leq \displaystyle (\frac{m}{5}+1)(\frac{m}{5}-2)<m+\frac{3}{2}\)
整理化簡後可得 \(\displaystyle 75\leq m^2-30m<87.5\)
\(\displaystyle m^2-30m=75,76,\cdots ,86,87\),利用同餘篩選並檢驗發現沒有整數解
故綜合以上,\(\displaystyle x=\frac{32}{5},x=\frac{-2}{5}\)