Complex sentence: For the left Riemann sum, approximating the function by its value at the left-end point gives multiple rectangles with base "Q" and height "f" ("a " +"iQ").
Split into simpler sentences: For the left Riemann sum, we will approximate the function by its value at the left-end point. This gives multiple rectangles with base "Q" and height "f" ("a " +"iQ").