ARITHMETIC CHECK: A sequence is said to be arithmetic if any two consecutive terms differ by the same constant.
So, the test to check if a series is arithmetic is to compute consecutive differences, and see if they all return the same number.
If we subtract the first two terms, we have
. If we subtract the third and second terms, we have
.
These two differences returned two different values, so the series is not arithmetic.
GEOMETRIC CHECK: A sequence is said to be geometric if any two consecutive terms are in the same ratio.
So, the test to check if a series is geometric is to compute consecutive ratios, and see if they all return the same number.
If we divide the first two terms, we have
![\dfrac{2}{4} = \dfrac{1}{2}](https://tex.z-dn.net/?f=%20%5Cdfrac%7B2%7D%7B4%7D%20%3D%20%5Cdfrac%7B1%7D%7B2%7D%20)
If we divide the third and second terms, we have
![\dfrac{1}{2} = \dfrac{1}{2}](https://tex.z-dn.net/?f=%20%5Cdfrac%7B1%7D%7B2%7D%20%3D%20%5Cdfrac%7B1%7D%7B2%7D%20)
Finally, if we divide the last two terms we have
![\dfrac{\frac{1}{2}}{1} = \dfrac{1}{2}](https://tex.z-dn.net/?f=%20%5Cdfrac%7B%5Cfrac%7B1%7D%7B2%7D%7D%7B1%7D%20%3D%20%5Cdfrac%7B1%7D%7B2%7D%20)
So, all ratios return the same number. This means that the series is geometric, and the common ratio is 1/2