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

#1**0 **

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**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