The approximate equation for f in the quadratic approximation method becomes undefined when K() = 1

The approximate equation for f in the quadratic approximation method becomes undefined when K(τ) = 1 − e^−rτ = 0, which corresponds to r = 0. Following a similar derivation procedure as in the quadratic approximation method, solve approximately the American option valuation problem for this degenerate case of zero riskless interest rate.