65/x=（2x+30)/100即：x(2x+30)=6500 2x^2+30x-6500=0 (2x-100)(x+65)=0所以 x=50或x=-65
65/x=(2x+30)/100x^2+15x-3250=0(x-50)(x+65)=0x=50或-65
