①2a²b-6ab² =2ab*a-2ab*3b=2ab*(a-3b)
②4m²-n² =(2m)²-n² =(2m+n)*(2m-n)
③25-4x²y² =5²-(2xy)²=(5+2xy)(5-2xy)
④m²-2分之1m+16分之1=m²-2*m*1/4+(1/4)²=(m-1/4)²