[500-10（x-50)](x-40)=8000(500-10x+500)(x-40)=8000(1000-10x)(x-40)=80001400x-10x²-40000=8000-10x²+1400x-48000=0-x²+140x-4800=0x²-140x+4800=0(x-60)(x-80)=0x=60 或 x=80
