(x-40)(240-3x)=900(x-40)(80-x)=30080x-x方-3200+40x=300-x方+120x-3500=0方法一：十字相乘法（x-50）(-x+70)=0x1=50 x2=70方法二：配方法-x方+120x-3600+100=0（x-60）方=100x-60=正负10x1=50 x2=70...