SOLUTION: A sequence of 20 terms is formed by using f(x)=4x-1 for x=1,2,3,...,20. What is the sum of the last eight terms. i just did 20 + 19 + 18 + ... 13 and it was wrong can you please

Question 550252: A sequence of 20 terms is formed by using f(x)=4x-1 for x=1,2,3,...,20. What is the sum of the last eight terms.
i just did 20 + 19 + 18 + ... 13 and it was wrong can you please explain how to do it
Answer by josmiceli(19441) (Show Source): You can put this solution on YOUR website!
You are correct that the last 8 terms are
term numbers 13, 14, 15, . . . 20
= term number, and
= the value of that term

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The sum would be
4*( 13 + 14 + 15 + . . . 20 ) - (1*8)
Note that
13 + 14 + 15 + . . . 20 = sum( 1 - 20 ) - sum( 1 - 12 )
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There is a general formula for finding the sum of ( 1 - n )
For example Sum( 1 - 5 ) = 15
This is
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Sum( 1 - 20 ) =
Sum( 1 - 12 ) =
Sum ( 13 - 20 ) =
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The sum of the last 8 terms is 520