(20+2x)(40-x)=1200 -(2x+20)(x-40)=1200 -(x+10)(x-40)=600 -(x?-30x-400)=600 -30x-400=-600 -30x+200=0
怎样把(20+2x)(40-x)=1200变形为x平方-30x+200=0
(20+2x)(40-x)=1200 -(2x+20)(x-40)=1200 -(x+10)(x-40)=600 -(x?-30x-400)=600 -30x-400=-600 -30x+200=0