SOLUTION: The 12th term of an arithmetic seqence progression is 5.the 7th term of this progression is 9 more than the 4th term. Determine the sum of 20 terms of this progression
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Question 1069082: The 12th term of an arithmetic seqence progression is 5.the 7th term of this progression is 9 more than the 4th term. Determine the sum of 20 terms of this progression Answer by ikleyn(52788) (Show Source):
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The 12th term of an arithmetic sequence progression is 5. the 7th term of this progression is 9 more than the 4th term.
Determine the sum of 20 terms of this progression
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"the 7th term of this progression is 9 more than the 4th term." means
= 9. (1)
But, as you know, = , = , therefore,
= 6d - 3d = 3d = 9.
Hence, d = 3.
Now you can determine the first term using the condition
"The 12th term of an arithmetic seqence progression is 5.".
It gives you an equation
= 5,
which implies = 5 - 11*3 = 5 - 33 = -28.
Having and d, you can calculate the sum of 20 terms of this progression using the formula
= = = 10.
