General form of a left-endpoint Riemann sum:
Hint for choosing the number of rectangles, n: While you want to choose a large value for n, be mindful that you will use that value to compute Δx.
Since b − a = 8, it is recommended (though not necessary) that you choose an n that is divisible by 8.
You should have recognized that this function is a semi-circle with center at the origin and radius 4. The exact area can be found with geometry.
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Click the link at right for the full version of the eTool: Calc 3-74 HW eTool