B.2y的平方+(-2y+1)=2y的2次方-2y+1
对
5x-(2x-1)-x²=5x-2x+1+x² ,×
=5x-2x+1-x²
3xy-0.5(xy-y²)=3xy-0.5xy+y²
×
3xy-0.5xy+0.5y²