x(20一x)=69 -x^2+20x-69=0 x^2-20x+69=0 x^2-20x+100-100+69=0 (x-10)^2=31 x-10=±根号31 x=±根号31+10
x^2 +20x-69=0(x+23)(x-3)=0x=-23 or 3
如图
