How do I solve this?

(3)16 Yiming has a great idea. He approximates .l: f(.'.tf} do: by 12 rectangles. In order to mitigate the error of left and right hand approximations, he takes the right endpoint of the first subinterval a5 a test point, but the left endpoint of the second subinterval. He continues to alternate for all 12 subintervals. What is another name for the approximation Yiming has produced