Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2005/ia/paper-1/5c/solution

For the unheaded continuation, set in the food equation. The two square roots are , so exponentiating gives
Both values have and hence lie strictly inside the disk. Both also have the indicated principal logarithms. Allowing the multivalued complex logarithm produces no additional points: the only permitted values of are still the two square roots already used.
For drink, the PDF squares only the denominator. Put . Its numerator is and its unsquared denominator is , which is nonzero unless . Multiplying the equation by this denominator squared and equating the real part and imaginary part yields
that is and . If , the real equation gives or , and zero is excluded. If , then , but the real equation would require , impossible for real . Thus
It lies on the allowed disk boundary, and direct substitution gives . The denominator-only square matters: squaring the entire quotient would define a different locus.
Figure 1.
Food points inside the allowed complex disk and the unique drink point on its boundary
. Details.

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