The 1st or x-coordinate evaluates to the the right endpoint of each subinterval. For example, the 1st subdivision is 0<=t<=1/n ---> 0<=nt<=1 ---> ceil(nt)=1 ---> (1/n)ceil(nt) = 1/n, the x-value at the right endpt of the first subinterval. The key is that the ceil( ) function replaces ALL t-values in that subinterval by a constant!