Question : average value of a function

Hi

In the graph below the average value of the function is 18 and the area below the curve is equal to the area above the curve. Why is area below equal to area above? It makes sense on an intuitive level but it would be nice to understand why with some sort of informal proof/explanation

thanks

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area.jpg (13 KB) (File Type Details) 

area

Answer : average value of a function

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