ax²＋bx＋c
＝a(x²＋bx/a)＋c
＝a(x²＋bx/a＋b²/4a²-b²/4a²)+c
＝a(x＋b/2a)²-b²/4a＋c
＝a(x＋b/2a)²＋(4ac-b²)/4a