@0celo7: "
How would you express the average speed without using intervals?" -- Right on. Yet there seems no explicit mentioning of intervals under
{tag:speed}; and it has even been claimed that
"speed itself is a coordinate system dependent concept". Btw., I'd express e.g.: $$c~\beta_{PQ}[~A~] = c~\sqrt{\frac{s^2[~\varepsilon_{AP},\varepsilon_{AQ}~] - s^2[~\varepsilon_{FQ},\varepsilon_{AQ}~]}{s^2[~\varepsilon_{AP},\varepsilon_{AQ}~]+s^2[~\varepsilon_{FQ},\varepsilon_{AQ}~]}},$$ [contd.] —
user12262 8 hours ago