х²-у²=72,
х+у=9
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(х-у)(х+у)=72,
х+у=9
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9(х-у)=72
х+у=9
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х-у=8
х+у=9
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х=8+у
8+у+у=9
2у=1
у=1/2 х=8+1/2=8,5
Ответ : х=8.5 у=0.5