①25X²+20XY+4Y²=(5x+2y)²②4x^4-4x^3+x^2=x²(4x²-4x+1)=x²(2x-1)²③(x-1)(x-3)+1=x²-4x+3+1=x²-4x+4=(x-2)²④(ab+a)+(b+1)=ab+a+b+1=(a+1)(b+1)