21012中２・式の計算・計算問題・同類項をまとめる2

$\begin{eqnarray}(1)\quad\;\;2.4a-0.5-1.1b+1.2+0.7b-1.8a\end{eqnarray}\;\;$

$\quad\;\;\begin{eqnarray}&=&2.4a-1.8a-1.1b+0.7b-0.5+1.2\\[6pt]&=&0.6a-0.4b+0.7\end{eqnarray}\;\;$

答$0.6a-0.4b+0.7$

$\begin{eqnarray}(2)\quad\;\;-\dfrac{1}{\;2\;}x^{2}+\dfrac{1}{\;2\;}x-\dfrac{1}{\;2\;}-\dfrac{1}{\;3\;}x+\dfrac{1}{\;3\;}+\dfrac{1}{\;3\;}x^{2}\end{eqnarray}\;\;$

$\quad\;\;\begin{eqnarray}&=&-\dfrac{1}{\;2\;}x^{2}+\dfrac{1}{\;3\;}x^{2}+\dfrac{1}{\;2\;}x-\dfrac{1}{\;3\;}x-\dfrac{1}{\;2\;}+\dfrac{1}{\;3\;}\\[6pt]&=&-\dfrac{3}{\;6\;}x^{2}+\dfrac{2}{\;6\;}x^{2}+\dfrac{3}{\;6\;}x-\dfrac{2}{\;6\;}x-\dfrac{3}{\;6\;}+\dfrac{2}{\;6\;}\\[6pt]&=&-\dfrac{1}{\;6\;}x^{2}+\dfrac{1}{\;6\;}x-\dfrac{1}{\;6\;}\end{eqnarray}\;\;$

答$-\dfrac{1}{\;6\;}x^{2}+\dfrac{1}{\;6\;}x-\dfrac{1}{\;6\;}$

$\begin{eqnarray}(3)\quad\;\;200b^{2}-120ab+400a^{2}+320ab-500a^{2}+100b^{2}\end{eqnarray}\;\;$

$\quad\;\;\begin{eqnarray}&=&400a^{2}-500a^{2}-120ab+320ab+200b^{2}+100b^{2}\\[6pt]&=&-100a^{2}+200ab+300b^{2}\end{eqnarray}\;\;$

答$-100a^{2}+200ab+300b^{2}$

$\begin{eqnarray}(4)\quad\;\;-\dfrac{2}{\;5\;}x^{2}-4.5xy+\dfrac{3}{\;5\;}y^{2}-0.6y^{2}-\dfrac{1}{\;2\;}xy+2.4x^{2}\end{eqnarray}\;\;$

$\quad\;\;\begin{eqnarray}&=&-\dfrac{2}{\;5\;}x^{2}+2.4x^{2}-4.5xy-\dfrac{1}{\;2\;}xy+\dfrac{3}{\;5\;}y^{2}-0.6y^{2}\\[6pt]&=&-\dfrac{2}{\;5\;}x^{2}+\dfrac{\;12\;}{5}x^{2}-\dfrac{9}{\;2\;}xy-\dfrac{1}{\;2\;}xy+\dfrac{3}{\;5\;}y^{2}-\dfrac{3}{\;5\;}y^{2}\\[6pt]&=&\dfrac{\;10\;}{5}x^{2}-\dfrac{\;10\;}{\;2\;}xy+0\\[6pt]&=&2x^{2}-5xy\end{eqnarray}\;\;$

答$2x^{2}-5xy$