1.先化简再求值.[(x+1/2y)^2+(x-1/2y)^2](2x^2-1/2y^2),其中x=-3,y=42.小明

[(x+1/2y)^2+(x-1/2y)^2](2x^2-1/2y^2),其中x=-3,y=4
[(x+1/2y)^2+(x-1/2y)^2](2x^2-1/2y^2)
=(x^2+xy+1/4y^2+x^2-xy+1/4y^2)(2x^2-1/2y^2)
=(2x^2+1/2y^2)(2x^2-1/2y^2)
=4x^4-1/4y^4
当x=-3,y=4时
原式=4*(-3)^4-1/4*4^4
=324-64
=260
2.小明在化简A-(ab+2bc-4ac)时,由于马虎,将"A-"写成"A+",得到的结果是3ab-2ac+5bc,问:正确的结果是多少?
A+(ab+2bc-4ac)=3ab-2ac+5bc
A=2ab+2ac+3bc
所以
A-(ab+2bc-4ac)
=2ab+2ac+3bc-ab-2bc+4ac
=ab+6ac+bc

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