a” - a - 12
= a” - 4a + 3a - 12
= a( a - 4 ) + 3( a - 4 )
= ( a - 4 )( a + 3 )
或者
= a” + 3a - 4a - 12
= a( a + 3 ) - 4( a + 3 )
= ( a + 3 )( a - 4 )
或者配方
= (1/4)[ 4a” - 4a - 48 ]
= (1/4)[ (2a)” - 2(2a) + 1 - 49 ]
= (1/4)[ ( 2a - 1 )” - 7” ]
= (1/4)( 2a - 1 - 7 )( 2a - 1 + 7 )
= (1/4)( 2a - 8 )( 2a + 6 )
= ( a - 4 )( a + 3 )
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