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1 vote
The first term of an arithmetic sequence is -30 and the common difference is 8. Find the 20:n term.

2 Answers

4 votes
the answer is 15! :)
User Protometa
by
8.9k points
3 votes

Answer:

n = 15

Step-by-step explanation:

This is an

Arithmetic sequence

The sum of the first n terms of this sequence is given by.

S

n

=

n

2

[

2

a

+

(

n

1

)

d

]

where a is the first term and d , the common difference.

here a = -16 , d = 8 and require to solve for n.

hence :

n

2

[

(

2

×

16

)

+

8

(

n

1

)

]

=

600

n

2

[

32

+

8

n

8

]

=

600

n

2

(

8

n

40

)

=

600

distributing gives :

4

n

2

20

n

600

=

0

Equated to zero since this is a quadratic equation.

4

(

n

2

5

n

150

)

=

0

4

(

n

+

10

)

(

n

15

)

=

0

n

=

10

or

n

=

15

but n > 0 hence n = 15

Step-by-step explanation:

User Jyo The Whiff
by
7.0k points

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