SOLUTION: There are 9 arithmetic means between 11 and 51. The sum of the progression is

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Question 817762: There are 9 arithmetic means between 11 and 51. The sum of the progression is
Answer by Edwin McCravy(20064) About Me  (Show Source):
You can put this solution on YOUR website!
There are 9 arithmetic means between 11 and 51. The sum of the progression is

11, _, _, _, _, _, _, _, _, _, 51

Notice with 9 terms between them, that 
gives a sequence of 11 terms, counting 
the 9 and the 51, so n=11, a1=11,
a11 = 51.  (There are two elevens, 
so don't get them mixed up :). 

If we use this sum formula, we don't need d:

Sn = n%2F2(a1 + an)

S11 = 11%2F2(11 + 51)

S11 = 11%2F2(62)

S11 = 341


Edwin