Homework Help Forum: math

Posted by Ellie on Sunday, October 25, 2009 at 9:56pm.

For an arithmetic sequence, a_{19} = 78. If the common difference is -3, find a_1 = (i found it to be 132) the sum of the first 57 terms =?? -I know there is a formula but i cant think of what it is

• math - Reiny, Monday, October 26, 2009 at 12:11am

  Sumn = (n/2)[2a + n-1)d]

  
  

• math - Reiny, Monday, October 26, 2009 at 12:12am

  Sumn = (n/2)[2a + (n-1)d]

  
  

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