Quick Answer SSS
Proof
AC = DB
AB = DC
BC = BC This side is equal to itself and is common to both triangles.
Three sides of one triangle equal to 3 sides of the other means that the triangles are congruent. It is the Theorem you need.
The expanded form is adding all the digits so the answer is
700+50+7+0.9+0.02+0.003
7/10 you just get the decimal form of the numbers and find a simple decimal between them, which was 7, and put it over 10 so you get 7/10
The initial term of this geometric series is 4, and the common ratio is -1/2. The sum is given by

The sum is
approximately 2.66.