Posted by **sdgsd** on Thursday, November 19, 2009 at 1:39pm.

An arithmetic series has third term 11. The ninth term is 5 times the second term. Find the common difference.

- math -
**drwls**, Thursday, November 19, 2009 at 2:10pm
Let the first term be called a and the common difference be called b.

a + 2b = 11

a + 8b = 5*(a + b) = 5a + 5b

The last equation can be rewritten

4a - 3b = 0

4a + 8b = 44

11b = 44

b = 4

a = 11 - 2b = 3

The series is 3,7,11,15,19,23,27,31,35...

Note that 35 (the ninth term) is 5 times 7

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