The centre distance between two meshing involute gears is
1
\(\frac{{sum~of~base~circle~radii}}{{cos~\phi}}\)
2
\(\frac{{sum~of~base~circle~diameters}}{{cos~\phi}}\)
3
\(\frac{{sum~of~base~circle~radii}}{{sin~\phi}}\)
4
\(\frac{{sum~of~pitch~circle~radii}}{{cos~\phi}}\)