Hello
The difference is whether the payoff on the derivative is based on the full range of values of the share price at expiry (ie ST from 0 to infinity) or just a truncated range (ie ST>K only). I'll try and explain below with a couple of examples.
All three questions (17.3, 17.4, 17.5) involve using the result of the five-step proof:
Vt = exp{-r(T-t)}EQ[XT|Ft]
where XT is the payoff on the derivative at time T.
In Q17.3(i) the payoff is: XT = ST - K if ST > K
In Q17.4(ii)(a) the payoff is: XT = 1 if ST < K
In Q17.4(ii)(a) the payoff is: XT = ST if ST < K
In Q17.5(i)(a) the payoff is: XT = ST^0.5 - K^0.5 for all values of ST
We can see that the range of values of T is truncated for Q17.3 and Q17.4 but not for Q17.5.
We can see above that the five-step proof result involves working out the expectation of the payoff. Typically, we calculate expectations of continuous random variables, by integrating, eg:
E[X] = ∫xf(x)dx
E[X^0.5] = ∫[x^0.5]f(x)dx
In Questions 17.3, 17.4, 17.5, the random variable is ST and hence we integrate over the range of values that ST can take.
In Q17.3(i), we would be integrating from K to ∞.
In Q17.4(ii)(a) and (ii)(b), we would be integrating from 0 to K.
In Q17.5(i)(a), we would be integrating from 0 to ∞.
In all three questions, ST|Ft has a logN(logSt + (r - 0.5σ^2)(T-t), σ^2)(T-t)) distribution.
This is the case, since the questions tell us that the share price follows geometric BM or the BS assumptions (of which geometric BM is one of the assumptions).
All three of the questions could be done using the formulae on page 18 of the Tables.
In Q17.3(i), we would take U = ∞ and L = K.
In Q17.4(ii)(a), we would take U = K and L = 0.
In Q17.4(ii)(a), we would take U = K and L = 0.
In Q17.5(i)(a), we would take U = ∞ and L = 0.
However, for Q17.5(i)(a), since we know we are integrating from 0 to ∞, ie over the full range of values, rather than over a range truncated at K, it is a lot quicker just to use the short-cut formulae for E[X^0.5] for the lognormal distribution on Page 14 of the Tables.
Does this make sense?
It's a useful exercise to try Q17.5(i)(a) using the Page 18 formulae instead, ie with k=0.5. Note that, in the calculation of Uk and Lk we would assume that log ∞ = ∞ and log 0 = -∞. (It's useful to draw the graph of log x vs x to see why this is case.)
Anna