当 i = 12% 时,
fnpv = 16309 (万元)
当 i = 15% 时,
fnpv = -8935×(p/f,15%,1)-24570×(p/f,15%,2)- 11164×(p/f,15%,3)+ 2549×(p/f,15%,4)+ 9078×(p/f,15%,5)+ 11278×(p/f,15%,6)+…+21924×(p/f,15%,18) = 6349(万元)
当 i = 18% 时,
fnpv =-8935×(p/f,18%,1)-24570×(p/f,18%,2)-11164×(p/f,18%,3)+ 2549×(p/f,18%,4)+ 9078×(p/f,18%,5)+ 11278×(p/f,18%,6)+…+21924×(p/f,18%,18) =-553(万元)
当 i = 12% 时,
fnpv = 16309(万元)
当 i = 15% 时,
fnpv = 6349(万元)
当 i = 18% 时,
fnpv =-553(万元)
∴ firr =15%+(18%-15%)
*6349/(6349+553) =17.72%