Question 1023337
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What is the sum of all of the odd numbers from 21 to 39?
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Use the formula 

1 + 3 + 5 + 7 + . . . + (2n-1) = {{{n^2}}}.

In your case 

1 + 3 + 5 + 7 + . . . + 39 = {{{20^2}}},

1 + 3 + 5 + 7 + . . . + 19 = {{{10^2}}},

so

the difference = {{{20^2 - 10^2}}} = 400 - 100 = 300.

It is exactly the sum 21 + 23 + . . . + 39, and it is your answer.
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