(0,7-x)^2=0,7^2-2*0,7*x+x^2=x^2-1,4x+0,49
(3x-1/6)^2=(3x)^2-2*3x*1/6+(1/6)^2=9x^2-x+1/36
(5xy+1)^2=(5xy)^2+2*5xy*1+1^2=25(xy)^2+10xy+1
(2/3-3)^2=(2/3)^2-2*2/3*3+3^2=4/9-4+9=5+4/9 (5 4/9)
