1 / f = 1 / a + 1 / b

That equation can represent the relationship among the image distance,
object distance, and focal length of a lens. 

It could describe the effective resistance of two resistors in parallel, the
effective inductance of two coils in parallel, or the effective capacitance
of two capacitors in series. 

Thanks for sharing it.  Now, did you have a question to ask ?

You have 2 sides that are 2 cms and you have 6 sides that are 1/2 cm and you have 4 sides that are 1 cm. the smallest sides are 1/2 the medium sized sides are 1 and the smallest are 1/2. so to find the perimeter you have to add them all up together so your answer will be 11 cm's