提公因式.x^2y^2-x^2(y-1)^2（a+b+c）^2-（a+b-c）^2

x^2y^2-x^2(y-1)^2
=x^2[y^2-(y-1)^2]
=x^2[y-(y-1)][y+(y-1)]
=x^2[y-y+1][y+y-1]
=x^2(2y-1)
（a+b+c）^2-（a+b-c）^2
=[(a+b+c)+(a+b-c)][(a+b+c)-(a+b-c)]
=[a+b+c+a+b-c][a+b+c-a-b+c]
=2c(2a+2b)
=4c(a+b)