十字相成例:6x²+19x-7=0(3x-1)(2x+7)=0x = 1/3 / x = -7/2配方法x²+4x-1=0x²+4x+2²-2²-1=0(x+2)²-2²-1=0(x+2)²=5x=±√5 -2求根公式法ax²+bx+c=0x = (-b±√b²-4...