Rút gọn biểu thức: (a+b-c)^2 +(a-b+c)^2 -2(b-c)^2

a+b−c2+a−b+c2−2b−c2=a2+b2+c2+2ab−2ac−2bc+a2+b2+c2−2ab+2ac−2bc+2b2+c2−2bc=2a2