EWt-n= A * ((1+i)n-1)/(1-(1/(1+i))
EWt-n/ A = ((1+i)n-1)/(1-(1/(1+i))
(EWt-n/ A) * (1-(1/(1+i)) = ((1+i)n-1)
1 + (EWt-n/ A) * (1-(1/(1+i)) = (1+i)n logarithmieren
log( 1 + (EWt-n/ A) * (1-(1/(1+i)) ) = n * log(1+i)
log( 1 + (EWt-n/ A) * (1-(1/(1+i)) ) / log(1+i) = n