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2015-06-05T22:50:40+08:00

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We can determine from here that the common difference is -5 since:
a_2-a_1=2-7=-5
so:
n= \frac{a_n-a_1}{d} +1= \frac{-18-7}{-5} +1= 6
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2015-06-06T01:48:39+08:00

This Is a Certified Answer

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Certified answers contain reliable, trustworthy information vouched for by a hand-picked team of experts. Brainly has millions of high quality answers, all of them carefully moderated by our most trusted community members, but certified answers are the finest of the finest.
The formula for the arithmetic sequence:
a_n=a_1+(n-1)d

Find the common difference:
2-7=-5

Substitute -18 as a_n
-18=7-5(n-1)
Distribute
-18=7-5n+5
Add like terms
-18=5n+12
Subtract both sides by 12
-30=-5n
Divide both sides by -5
n=6

Hope this helps =)

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