It represents a geometric sequence because it has a common ratio of 2.
In fact, a sequence is said to be geometric if any two adjacent elements are in the same ratio. In other words, if you choose any index and consider the two consecutive terms
and
, you have
no matter which index you chose. In your case, the next term in the sequence is always the double of the previous one, so the ratio between two consecutive terms is always 2, and the series is geometric with common ratio 2
1. , then
and triangles ADC and ACB are similar by AAA theorem.
2. The ratio of the corresponding sides of similar triangles is constant, so
.
3. Knowing lengths you could state that .
4. This ratio is equivalent to .
5. , then
and triangles BDC and BCA are similar by AAA theorem.
6. The ratio of the corresponding sides of similar triangles is constant, so
.
7. Knowing lengths you could state that .
8. This ratio is equivalent to .
9. Now add results of parts 4 and 8:
.
10. c is common factor, then:
.
11. Since you have
.