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原式=b² - (2/3)ab + a²/9 + 2(b/3)² + (2b/3)²
=b² - (2/3)ab + a²/9 + 2•(b²/9) + 4b²/9
=b² - (2/3)ab + a²/9 + 2b²/9 + 4b²/9
=(1 + 2/9 + 4/9)•b² - (2/3)ab + a²/9
=5b²/9 - (2/3)ab + a²/9
=b² - (2/3)ab + a²/9 + 2•(b²/9) + 4b²/9
=b² - (2/3)ab + a²/9 + 2b²/9 + 4b²/9
=(1 + 2/9 + 4/9)•b² - (2/3)ab + a²/9
=5b²/9 - (2/3)ab + a²/9
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