实数a

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实数a

( a" + b" ) / ( a + b )
= ( a" + 2ab + b" - 2ab ) / ( a + b )
= [ ( a + b )" - 2 ] / ( a + b )
= ( a + b )" / ( a + b ) - 2 / ( a + b )
= - ( a + b ) - 2/(a + b)