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Cloud [144]
3 years ago
5

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

Mathematics
2 answers:
jek_recluse [69]3 years ago
8 0

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















sladkih [1.3K]3 years ago
7 0

Answer:

The correct answer is d-71.3

Step-by-step explanation:

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