∵(x-50)[800-(x-60)/2*40]=12000∴(x-50)[800-20(x-60)]=12000,∴(x-50)(800-20x+1200)=12000,∴(x-50)(2000-20x)=12000,∴20(x-50)(100-x)=12000,∴(x-50)(100-x)=600,∴-x^2+150x-5000=600,∴-x^2+150x-5600=0,∴...