几道数学代数乘法公式题

1、(x+y)^2-4(x-y)^2
=(x+y)^2-[2(x-y)]^2
=(x+y+2x-2y)(x+y-2x+2y)
=(3x-y)(3y-x)
2、∵x+y=1,xy=1/2
∴x(x+y)(x-y)-x(x+y)²
=x(x+y)(x-y-x-y)
=x(x+y)(-2y)
=-2xy(x+y)
=-2*(1/2)*1
=-1
3、由题可知
∵草坪的面积＝空地面积-4*花坛面积
∴草坪的面积＝12.8^2-4*1.4^2
=(12.8+2*1.4)(12.8-2*1.4)
=(12.8+2.8)(12.8-2.8)
=15.6*10
=156(m²)
答：草坪面积156平方米