Question

Calculate the arithmetic sequence in which a9=17 and the common difference is d=-2.1

Answer 1

Answer:

$S_{31}=71.3$

Step-by-step explanation:

The nth term of an arithmetic sequence is given by the formula,

$U_n=a_1+(n-1)d$

We were given that the 9th term is $17$.

$\Rightarrow 17=a_1+(9-1)(-2.1)$

$\Rightarrow 17=a_1+(8)\times(-2.1)$

$\Rightarrow 17=a_1-\frac{84}{5}$

$\Rightarrow 17+\frac{84}{5}=a_1$

$\Rightarrow a_1=\frac{169}{5}$

The sum of the first n-terms is given by the formula,

$S_n=\frac{n}{2}(2a_1+(n-1)d)$

To find $S_{31}$, we substitute $n=31$, $a_1=\frac{169}{5}$ and $d=-2.1$.

$\Rightarrow S_{31}=\frac{31}{2}(2(\frac{169}{5}+(31-1)(-2.1))$

$\Rightarrow S_{31}=\frac{31}{2}(2(\frac{169}{5}+(30)(-2.1))$

$\Rightarrow S_{31}=\frac{31}{2}(\frac{23}{5})$

$\Rightarrow S_{31}=\frac{713}{10}$

$\Rightarrow S_{31}=71.3$

The correct answer is D

Answer 2

Answer:

The correct answer is d-71.3

Step-by-step explanation: