解:
(1)
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=
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,
x=21×2,
x=42;
(2)6:x=9:24,
9x=6×24,
9x=144,
9x÷9=144÷9,
x=16;
(3)6:x=
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,
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x=6×
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,
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x÷
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=6×
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÷
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,
x=6×
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×
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,
x=10;
(4)6:9=
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:x
6x=9×
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,
6x=3,
6x÷6=3÷6,
x=
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;
(5)
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=
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,
8x=21×0.4,
8x=8.4,
8x÷8=8.4÷8,
x=1.05;
(6)
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:
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=x:9,
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x=
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×9,
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x=6,
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x÷
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=6÷
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,
x=
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.
分析:(1)根據(jù)比例的基本性質(zhì),兩內(nèi)項(xiàng)之積等于兩外項(xiàng)之積,把原式改寫成x=21×2,然后再進(jìn)一步解答即可;
(2)根據(jù)比例的基本性質(zhì),兩內(nèi)項(xiàng)之積等于兩外項(xiàng)之積,把原式改寫成9x=6×24,然后等式的兩邊同時(shí)除以9即可;
(3)根據(jù)比例的基本性質(zhì),兩內(nèi)項(xiàng)之積等于兩外項(xiàng)之積,把原式改寫成

x=6×
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,然后等式的兩邊同時(shí)除以
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即可;
(4)根據(jù)比例的基本性質(zhì),兩內(nèi)項(xiàng)之積等于兩外項(xiàng)之積,把原式改寫成6x=9×
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,然后等式的兩邊同時(shí)除以6即可;
(5)根據(jù)比例的基本性質(zhì),兩內(nèi)項(xiàng)之積等于兩外項(xiàng)之積,把原式改寫成8x=21×0.4,然后等式的兩邊同時(shí)除以8即可;
(6)根據(jù)比例的基本性質(zhì),兩內(nèi)項(xiàng)之積等于兩外項(xiàng)之積,把原式改寫成
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x=
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×9,然后等式的兩邊同時(shí)除以
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即可.
點(diǎn)評(píng):本題主要考查解比例,根據(jù)比例的基本性質(zhì)和等式的性質(zhì)進(jìn)行解答即可.