第10页

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$\begin{aligned}\frac{5}{9}÷4+\frac{3}{4}×\frac{5}{9}&=\frac{5}{9}×\frac{1}{4}+\frac{3}{4}×\frac{5}{9}\\&=\frac{5}{9}×(\frac{1}{4}+\frac{3}{4})\\&=\frac{5}{9}\end{aligned}$
$\begin{aligned}\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+\frac{1}{16}+\dots+\frac{1}{1024}&=\frac{3}{4}+\frac{1}{8}+\frac{1}{16}+\dots+\frac{1}{1024}\\&=\frac{7}{8}+\frac{1}{16}+\dots+\frac{1}{1024}\\&=\frac{1023}{1024}\end{aligned}$
$\begin{aligned}4.3×81+1.9×43&=43×8.1+1.9×43\\&=43×(8.1+1.9)\\&=43×10\\&=430\end{aligned}$
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