(4+y^2)*(y^2-4)= -16+y^4; 25x^2-y^2=(5x-y)*(5x+y); (9-y)^2=81-18y+y^2; b^2+4a^2-4ab=(b-2a)^2; (b+3)^2=b^2+6b+9; 1+x^2-2x=(1-x)^2; (2x-1)(2x+1)=4x^2-1; 28xy+49x^2+4y^2=(7x+2y)^2; (7x-2)(7x+2)=49x^2-4; (a+1)^3=a^3+3a^2+3a+1; (2-a)^3=8-12a+6a^2-a^3. (k+0,5)^2=k^2+k+0,25.