(50-40+x)(500-10x)=800(10+x)(500-10x)=800(10+x)[10*(50-x)]=10*80(10+x)(50-x)=8010*50-10*x+x*50-x*x=80500-10x+50x-x^2=80500+40x-x^2=80500-80+40x-x^2=0420+40x-x^2=0x^2-40x-420=0x^2-2*x*20-420=0x^2-2*x*2...