Sigma notation with area under a curve, lower and upper rectangles
I got the answer to this problem from my teacher but I don't understand why that's the answer:
y=√1+x^3 , domain 1≤x≤2 and with 50 intervals therefore width is 1/50.
The Sigma notation for the lower rectangle is (1/50)∑√(1+(x/50)^3) with a 99 written above Σ and the what's written below Σ being x=50. Why isn't the upper limit n-1 aka 49 and the lower 0? Is my teacher wrong or am I misunderstanding sigma notation?