-2000+500(1+r)^-1+700(1+r)^-2+2000(1+r)^-3<0
    310x+460(50-x)≤18000
    （72+86+x)/3≥80
    (5x-1)/3≤x+1
     -2000+500/(1+r)+700/(1+r)^2+2000/(1+r)^3>-10000+8000/(1+r)+2500/(1+r)^2+2000/(1+r)^3
    -2000 + 500/(1 + r) + 700/(1 + r)^2 +2000/(1 + r)^3>-10000 + 8000/(1 + r) +2500/(1 + r)^2 + 2000/(1 + r)^3
    -10000x^3+8000x^2+2500x+2000>0
    -2000+500/1+x+700/(1+x)^2+2000/(1+x)^3>-10000+8000/1+x+2500/(1+x)^2+2000/(1+x)^3
    80x^2+85x-13>0
    80x^2+157x-13>0
    8000>300/(1+x)+1800/(1+x)^2
    80x^2+157x+59>0
    p*p-2*p<8
    p*p-3*p>0
    p*p-p>2
    80r^2+157r+59＞0
    -20*(1+a)^3+5*(1+a)^2+7*(a+1)+20>0
    8000(1+i)^2-7500(1+i)-1800>0
    8000x^2-7500x-1800>0
    8000x^2-75x-18>0

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