(1000-10x)*(x-40)=8000

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(1000-10x)*(x-40)=8000

(1000-10x)*(x-40)=8000
(1000-10x)*(x-40)=8000

(1000-10x)*(x-40)=8000
两边除以-10
(x-100)(x-40)=-800
x²-140x+4000=-800
x²-140x+4800=0
(x-60)(x-80)=0
x=60,x=80

1000x-40000-10x²+400x=8000
10x²-1400x+48000=0
x²-140x+4800=0
x²-140x+4900-4900+4800=0
(x-70)²=100
x-70=±10
x=80或60