Hi,

sum of the first twenty terms of the sequence 3,5,7,........

 = 10(6+38) = 440 

Answer by jrfrunner(365)   (Show Source): You can put this solution on YOUR website!
 In the sequence you provide each number changes by an "constant" value. 

This is known as an arithmetic sequence 

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The change, or  is the difference between any two values, so in this case =2

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For an arithmetic sequence, the nth term =

and for an arithmetic series, its sum is defined as the average value times the number of terms or Sum=

==

in the sequence you provide, =3 and the last term 

--

The sum = average value * n =  

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