maths 题

492200= 125000+125000/ (1+i) + 125000/ (1+i) 2+200000/ (1+i) 3+80000/ (1+i) 3
4922 = 1250 + 1250/(1+i) + 1250/ (1+i)^2+2000/ (1+i)^3 + 800/ (1+i)^3
3672 = 1250/(1+i) + 1250/(1+i)^2 + 2800/ (1+i)^3
1836 = 625/(1+i) + 625/(1+i)^2 + 1400/(1+i)^3
設 y = 1/(1+i) :
1400y^3 + 625y^2 + 625y - 1836 = 0
用這個程式 :
http://www.easycalculation.com/algebra/cubic-equation.php
實數解 1/(1+i) = 0.8486032
i = 0.178407

i= -2591/11016