Answer
See below.
Work Step by Step
In order for a sequence to be geometric, the quotient of all consecutive terms must be constant.
Hence here: $1.5/3=0.5=0.75/1.5=r$, thus it is a geometric sequence.
Hence it is a geometric sequence.
The sum of a geometric sequence until $n$ can be obtained by the formula $S_n=a_1(\frac{1-r^n}{1-r})$ where $a_1$ is the first term and $r$ is the common ratio. Hence here the sum is: $S_n=3(\frac{1-0.5^n}{1-0.5})$.
