Question

Classify the series as arithmetic or geometric then determine whether the series is convergent or divergent

Answer 1

Answer:     Arithmetic , Divergent

Step-by-step explanation:

Given sequence is  

{aₙ} =  {  4 , 10/3  , 8/ 3 , 2 , . . .   }

To check whether the sequence is geometric or not , we divide second term by first term  to find the common ratio . Then we again divide third term by second term to get common ratio .

The common ratio we get would same , then it is geometric .

         10/3               10            5

r₁  =  -----------  =     ------    =   ------

             4                12             6

        8/3                8                  3                4

r₂=   ----------   =  ----------  *   ------------   =     -----

           10/ 3             3              10                  5

Thus the common ratio are not same . So the sequence is not geometric .

Now we check for arithmetic .

We take difference of second and first term and then difference of third and second term . If it will be same then it is arithmetic . This is called common difference , d .

             10                       -  2

d₁   =    ------     - 4   =       -------

              3                          3

                8            10             - 2

d₂   =    ------    -   --------    =   ---------

               3             3                 3

Thus the common difference is same .

So the given sequence is arithmetic .

To find whether it is convergent or divergent , we need to write sum of n terms first .

Formula for finding sum of n terms of arithmetic sequence is

            n

sₙ =     -----  [ 2a + ( n - 1 ) d]

             2

We have a = 4 , d = - 2/3 .

Plug  in this formula we get

              n                                                    n                2             2

sₙ  =     ------- [ 2 * 4 + ( n - 1 ) ( -2/3) ]  =     ------ [  8 -   -----  n  +   ------  ]

              2                                                    2                3               3

           n          26              2

sₙ  =   ------  [    ------    -    ------- n  ]

           2           3                3

To check whether it is convergent or divergent , we take limit sₙ approaches to infinity .

                               n       26       2

lim   sₙ   =     lim    { ---   [  ---  -  ------ n ] } =     - ∞

n → ∞           n→∞      2        3       3

As the sequence diverge , thus the series is divergent .

Thus given series is arithmetic , divergent .

Second is the  correct option .