Posted by **Ellie** on Sunday, October 25, 2009 at 8:45pm.

-Write the arithmetic sequence 21,13,5,-3... in the standard form:

a_n=

-a_n=a_1+(n-1)d--so a_n=21+(n-1)-8

*a_n=-168-8n

why isnt this right?

- math -
**Quidditch**, Sunday, October 25, 2009 at 9:00pm
You are close. You have an error:

Your step:

a_n = 21 + (n - 1) - 8

should be:

a_n = 21 + (n - 1) * (-8)

- math -
**Ellie**, Sunday, October 25, 2009 at 9:03pm
oh yeah sorry i wrote that wrong i put -168-8n but it still said my answer was wrong

- math -
**Quidditch**, Sunday, October 25, 2009 at 9:15pm
a_n = 21 + (n-1)*(-8)

Expand that using the distributive law.

The first term, 21, is not multiplied by -8. Only the terms in parenthesis are multiplied by -8.

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**Ellie**, Sunday, October 25, 2009 at 9:54pm
i figured out what i was doing thank you so much!

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**Quidditch**, Sunday, October 25, 2009 at 10:03pm
You're very welcome.

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**Edward**, Sunday, November 30, 2014 at 12:17am
arithmetic sequence

b_n=(-2)^(n-1)

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