M^2+3mn+n^2 ,(2n+3)^2 ,a^2-4a+4 (x-y)^2 a^2+2ab+b^2 p^2+4pq+q^2 (2+3k)^2 a^2+6a+9 (c+d)^2 1+2x+x^2 a^2+8a+16 (2p+q)^2 4a^2-4a+1 (a-b)^2 x^2-2xy+y^2 m^2-3mn+n^2 (2-3)^2 a^2-4a+4 (x-y)^2 a^2-2ab+b^2 p^2-4pq+q^2 (2-3k)^2 a^2-6a+9