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15)
Find the sum of the arithmetic series for which a1 = 2, an = 20, and n = 7.

Sagot :

Answer: -430

Step-by-step explanation: The formula for the sum of an arithmetic sequence when

a

,

d

and

n

are given is:

S

u

m

=

(

n

2

)

[

2

a

+

(

n

−

1

)

d

]

Here :

a

1

=

7

,

d

=

−

3

and

n

=

20

=

(

20

2

)

[

2

.

7

+

(

20

−

1

)

.

−

3

]

=

10

.

[

14

+

19

.

(

−

3

)

]

=

10

.

[

14

−

57

]

=

10

.

(

−

43

)

=

−

430

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