Which component of the atom has the least mass

In the figure below, segment ac is congruent to segment ab: triangle abc with a segment joining vertex a to point d on side bc. side ab is congruent to side ac which statement is used to prove that angle abd is congruent to angle acd? segment ad bisects angle cab. triangle acd is similar to triangle abd. segment ad is congruent to segment ac. angle cab is congruent to angle cba.

Consider the proof. given: segment ab is parallel to line de. prove: what is the missing statement in step 5?

Ihave trouble finding the nth term of a linear sequence