 Solution

ID: past-exam-of-the-mathematics-course-of-the-year-1-of-the-natural-sciences-course-of-the-university-of-cambridge/2019/paper-2/10/a/solution

For the outward orientation of a spherical cap, its vector area element is

d S = \hat{r} a^{2} sin θ d θ d ϕ, \hat{r} = (sin θ cos ϕ, sin θ sin ϕ, cos θ) .

x

and

y

components vanish by integration over a complete azimuth. The remaining component is

S_{z} = 2 π a^{2} \int_{0}^{π /4} cos θ sin θ d θ = \frac{π a ^{2}}{2} .

Consequently

S = (0, 0, \frac{π a ^{2}}{2}) .

This also equals the area of the cap's projection onto the horizontal plane, directed upwards.

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