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If the eight term of an arithmetic sequence with first term -1 is 20, then what is the common difference?

 

The first and last terms of an arithmetic sequence with thirteen terms are -7 and -115, respectively. Find the sum of the sequence.

 

Please answer both questions and explain the steps,

thankyou.

Guest Jul 24, 2017

Best Answer 

 #1
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1) F + (N - 1)*D=Nth Term, where F=First term, D=Common difference and N=Number of terms.

-1 + (8 - 1)*D =20, solve for D

-1 + 7D = 20

7D = 20 + 1

7D =21 divide both sides by 7

D = 21 / 7

D = 3 Common difference

 

2)

(F + L) / 2*N, where F=First term, L=Last term and N=Number of terms.

Sum = (-7 - 115) / 2*13

Sum =( -122 / 2) * 13

Sum = -61 * 13

Sum = -793

Guest Jul 24, 2017
 #1
avatar
0
Best Answer

1) F + (N - 1)*D=Nth Term, where F=First term, D=Common difference and N=Number of terms.

-1 + (8 - 1)*D =20, solve for D

-1 + 7D = 20

7D = 20 + 1

7D =21 divide both sides by 7

D = 21 / 7

D = 3 Common difference

 

2)

(F + L) / 2*N, where F=First term, L=Last term and N=Number of terms.

Sum = (-7 - 115) / 2*13

Sum =( -122 / 2) * 13

Sum = -61 * 13

Sum = -793

Guest Jul 24, 2017

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