（4a²+1）²-16a²=[(4a^2+1)+4a][(4a^2+1)-4a]=(4a^2+4a+1)(4a^2-4a+1)=(2a+1)^2*(2a-1)^24xy²-4x²y-y³ =-y(4x^2-4xy+y^2)=-y(2x-y)^2