解:①2.5:0.45=(2.5×100):(0.45×100)=250:45=(250÷5):(45÷5)=50:9;
②24:
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=(24×27):(
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×27)=648:8=(648÷8):(8÷8)=81:1;
③
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:
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=(
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):(
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)=1:2;
④
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:0.25=
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:
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=
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:
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=(
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):(
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)=3:1.
分析:根据比的基本性质(比的前项和后项同时乘上或除以一个相同的数(0除外),比值不变),把比的前项和后项同时乘上或除以一个相同的不为0的数,使比的前项和后项变成互质数.
点评:化简比是把一个比化成最简单的整数比(前项和后项是互质数)的形式.根据比的前后项,灵活运用所学的知识化简即可.