①(a+b)²-(a-b)²
=a²+2ab+b²-a²+2ab-b²
=4ab

②（a+b-c）²
=a²+b²+c²+2ab-2ac-2bc

③（x-y+z）（x+y+z)
=(x+z)²-y²
=x²+2xz+z²-y²