<s>[INST] Sum of even numbers N? To sum first N numbers we can use this formula:
$$1 + 2 + 3 + \ldots + n = \frac{(n  (n + 1)}{2}$$
To sum even numbers we multiply this formula by 2:
$$2 + 4 + 6 + \ldots + 2n = n (n + 1)$$ 
Lets check sum of even numbers until 6:
Sum(odd) =  2 + 4 + 6 = 12

Then let's use the formula: 6 * (6 + 1) = 6 * 6 + 6 = 42.
What is wrong with formula or my calculations? 
 [/INST] You use that
$$2+4 + 6+ \ldots + 2n = 2(1 + 2 + 3 + \ldots + n) = 2\cdot \frac{n(n+1)}{2} = n(n+1)$$
So the $n$ used is the number of terms added, not the final term, but half of it.
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