用心算一算:(1)2a 5 -(-a) 2 -(-a 2 ) 3 -(-7a);(2)(4x 2 y+5xy-7x)-(5x 2 y+4xy+x);(3)( 1 2 x 2 y-2xy+y 2 )-3xy;(4)(4x 3 y-6x 2 y 2 +12xy 3 )÷(2xy);(5)化简求值(x+2y) 2 -(x+y)(x-y),其中x=2,y= 1 2 .
(1)2a 5 ?(-a) 2 -(-a 2 ) 3 ?(-7a), =2a 7 -7a 7 =-5a 7 ; (2)(4x 2 y+5xy-7x)-(5x 2 y+4xy+x), =-x 2 y+xy-8x; (3)(
=
(4)(4x 3 y-6x 2 y 2 +12xy 3 )÷(2xy), =2x 2 -3xy+6y 2 ; (5)(x+2y) 2 -(x+y)(x-y), =x 2 +4xy+4y 2 -x 2 +y 2 , =4xy+5y 2 , 当x=2,y=
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