How can a mix of shares and bonds copy a call in both states, and why does the chance of the stock rising drop out of its price?
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The portfolio of shares plus bonds worth
Write its cost and its two payoffs and , recording that is the gross riskless return, a number like 1.03.
Solving for and
Set the two payoffs equal to and , solve the linear system to get and , and evaluate both in one numerical example.
The call price
State that the replicating portfolio's cost is the call price, substitute to reach , and finish the numerical example with its price.
The probability drops out
Record that appears nowhere in the formula, that investors must agree on and but may disagree about , and that this disagreement is what makes a market.