[10(9-x)+x]*[10x+(9-x)]=2430[(90-10x)+x]*[10x+9-x]=2430(90-9x)*(9x-9)=2430[9(10-x)]*[9(x-1)]=243081(10-x)*(x-1)=2430(10-x)*(x-1)=2430/81=30-x^2+11x-10=30x^2-11x+40=0判别式=b^2-4ac=121-160