原式=2A(X-2Y)^2+3B(X-2Y)^3—4C(X-2Y)^4
=(X-2Y)^2[2A+3B(X-2Y)-4C(X-2Y)^2]
=(X-2Y)^2[-4CX^2+16CXY+3BX-6BY-16Y^2+2A]