化简：16000-800x-200x+10x^2=9000
7000-1000x+10x^2=0
x^2-100x+700=0
x=(100±√10000-2800)/2=50±30√2
so:x1=50+30√2
x2=50-30√2
解方程（80-x)（200 10x）=9000
化简：16000-800x-200x+10x^2=9000
7000-1000x+10x^2=0
x^2-100x+700=0
x=(100±√10000-2800)/2=50±30√2
so:x1=50+30√2
x2=50-30√2