（0.3-x）（500+10x)=1200.3*500+0.3*10x-500x-10x²=120 150+3x-500x-10x²=120 -10x²-497x+150=120 -10x²-497x+30=0 10x+497x-30=0a=10,b=497,c=-30 ······
