初一下数学（1）（0.2x-0.3)(0.2+0.3) (2) (mn-3n)(mn+3n) (3) (-2x+3y)(

（1）（0.2x-0.3)(0.2x+0.3)=(0.2x)²-0.3²=0.04x²-0.09
(2) (mn-3n)(mn+3n)=(mn)²-(3n)²=m²n²-9n²
(3) (-2x+3y)(-2x-3y)=(-2x)²-(3y)²=4x²-9y²
(4) (-四分之一x-2y)(-四分之一x+2y)=(-四分之一x)²-(2y)²=十六分之一x²-4y²
(15) x^2+(y-x)(y+x)=x²+y²-x²=y²