My Battle Against Math!!!

[jetangel]

So I finally figured out two problems from my math homework from a week ago. >_< It's good that I don't have to turn in problems because it takes me FOREVER to do the simplest math! Basic algebra hates me. So thank GAWD and mostly Susan for solution manuals!!! Although i got the answer for one I didn't get what LCD was...perhaps it has something to do with physics??? If anyone gets this PLEASE tell me:

1   = 1   +   1   
 f      d1       d2        LCD = d1d2f     (simple lens formula)

SOLUTION: f = d1d2__
                   d2 + d1   
Does anyone get that at all??? I feel so stupid. >.