Dear all,

I am having trouble with the following exercise and would like to ask if the step underlined in red could be please explained.

I understand everything up to that point but don't understand why only is considered in the integral. What about ?

Also, what does the mean? Does it represent the fact that the wedge (highlighted in light green below in my graph) is multiplied by 16 times?

Thank you very much.

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Find the area of the region that lies inside both and .

Question:

Given solution:

My graph: