Question 371414
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 {{{delta}}} is the difference between any two values, so in this case {{{delta}}}=2
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For an arithmetic sequence, the nth term {{{a[n]}}}={{{a[1]+(n-1)*delta}}}
and for an arithmetic series, its sum is defined as the average value times the number of terms or Sum={{{((a[1]+a[n])/2)*n}}}
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in the sequence you provide, {{{a[1]}}}=3 and the last term {{{a[n]=3+(20-1)*2=41}}}
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The sum = average value * n = {{{((first + last)/2) *n =((3+41)/2)*20=440}}}