knowledge - Replace the question marks with one single word

This one needs a lot of lateral thinking

In the equation below replace all the question marks with one single word (same word at every question mark).

Then show that the equation is correct.

Answer

Use ?=ROT to get:

ROT(ROT5(68))[ROT5(105)]=ROT(ROT5(68))[P]/ROT(ROT5(68))[Q]

which becomes:

ROT13[ROT5(105)]=ROT13[P]/ROT13[Q]

Now the clever bit, 105=CV in Roman numerals, and P->C, Q->D in ROT13:

ROT13(CV)=C/D

PI=Circumference/Diameter