A math exercise can prove that the earth is flat. When you differentiate an equation....like Y=X^2 it becomes dy/dx=2X......you are actually finding the exact slope of two connected points on the equations curve. It is so small a space on the curve, that it is for all practical purposes a flat length of line. .........By extension, we are so tiny a location on the Cartesian graph of the greater universe that we just have to be living on a flat place. Just like the differential says.......... .......