Finding the sum of arithmetic sequence.?

Question

In exercises 1-5, find the sum of the arithmetic sequence having the data given. Project 8.02

a1 = 5, an = 3, n = 20

a1 = 7, d = - 3, n = 20

a1 = 81, an = - 13, n = 20

a1 = 5, d = 7, n = 120

an = 5, d = -2, n = 12

Answer 1

Use the formula:

Sn = ½n(a1 + an)

a1 = 5, an = 3, n = 20

Sn = ½20(5 + 3)

Sn = 10(8)

Sn = 80

a1 = 7, d = - 3, n = 20

an = -3n + 10

an = -3(20) + 10

an = -60 + 10

an = -50

Sn = ½(20)(7 + -50)

Sn = 10(-43)

Sn = -430

a1 = 81, an = - 13, n = 20

Sn = ½20(81 + -13)

Sn = 10(68)

Sn = 680

a1 = 5, d = 7, n = 120

an = 7n + -2

an = 7(120) + -2

an = 840 + -2

an = 838

Sn = ½(120)(5 + 838)

Sn = 60(843)

Sn = 50580

an = 5, d = -2, n = 12

an = -2n + 29

a1 = -2(1) + 29

a1 = 27

Sn = ½(12)(27 + 5)

Sn = 6(32)

Sn = 192

Hope that helps!