So if you check the picture below, each side of the diamond has 90ft, so half-way from first to second is 45ft over the "x" line
a)
in this case, notice that "y" is static, is a constant, it doesn't change, whilst the "r" distance is and the "x" as well, when the runner is going to 1st base
b)
well on this case, the distance "y" from home plate to 3rd base, isn't changing either, is a constant and is also 90ft as well