(X-15)²
=x^2-30x+225
（y－7﹚²
=y^2-14y+49
﹙3＋x﹚²
=x^2+6x+9
﹙2－y﹚²
=y^2-4y+4
﹙a＋b＋c﹚²
=(a+b)^2+2(a+b)c+c^2
=a^2+b^2+2ab+2ac+2bc+c^2
=a^2+b^2+c^2+2ab+2ac+2bc