點解 MR=MC 既時候 ~ PROFIT 就最高既 ???

First you have to be clear on the definitions of MR and MC
MR = dTR/dQ, MC = dTC/dQ
(ADDITIONAL revenue / costs that the production of one extra unit of output will bring)

That is because

if MR > MC,
then there is still room for profits and production of extra units will continue as long as this condition is met

if MR < MC, then there is no point to produce that extra quantity of unit as long as this condition is met
(e.g. if MC curve cuts MR curve from ABOVE at Q=5, then the firm will only start its production from Q=5, which might happen for things like power plants because of the large fixed costs involved)

For example, suppose when Q=2, MR = 5, MC=3, you produce this unit. When Q=3, MR=5, MC=7, you do not produce this unit because it will result a negative profit.

Therefore MR=MC is a condition when all opportunities for extra profit are exhausted and the current extra unit of production is already receiving zero profit.

MR係邊際的利益
MC係邊際的成本
MR 最後會不斷的下趺，而MC最後會不斷升
當MR = MC 時，其實已經賺盡了
再要下一個的時候，MR < MC,你會earn少左