16x2+*+y2=
49-*+x2=
a2+18a+*=
*-12x+9x2=

(a+b)²=a²+2ab+b²  (a-b)²=a²-2ab+b²16х²+8ху+у²= (4х+у)²49-14х+х²=(7-х)²а²+18а+81=(а+9)²4-12х+9х²=(2-3х)²