i need to know the formula for the surface area of a disk I am trying to figure out a questions but i need to know the formula for the surface area of a disk. if anyone could help me out it would be great.
The question i am trying to answer is: " the concrete pipe shown in the diagram has the following measurements: t(thickness of the pipe): 30mm d(diameter): 18cm and l(length):27cm.
i have to figure out the surface area of both sides of this pipe. the sides are shaped like disks.
sorry the diagram is upside down but its really hard trying to take a pic with a macbook


Lateral area of a cylinder of diameter $d$ and length $l$: $\pi d l$.
Area of a disk of diameter $d$: $\,\pi\dfrac{d^2}4$, hence the area of an annulus between a disk of diameter $d$ and a disk of diameter $d'$: $\dfrac{\pi(d^2-d'^2)}4$.
Total area, taking into account the inner cylinder:
$$\pi(d+d')l+2\pi\frac{d^2-d'^2}4=\pi(d+d')\Bigl(l+\frac{d-d'}2\Bigr).$$
Now $d'=d-2t$. There remains to simplify the results and compute numerically.
