20000+200x-50x-xx/2=30000150x-xx/2=10000300x-xx=20000xx-300x=-20000(x-150)(x-150)=2250x=150±15根号10
(400-x)(50+1/2x)=300002000-50x+200x-1/2xx=30000150x-1/2xx=-28000