fully expand your equations, c^2 = 49 * a^2

your soultion is correct and good  for training math research skill, but it is hard to do it in a contest with the time limit. Too many places can commit errors.

also, d is zero comes from 3rd order term in your expansion. No need to prove it by contradiction.

所有跟帖： 

• Thanks. -dr_yin- ♂ (0 bytes) () 10/17/2015 postreply 16:18:24