On the 12th day, the true love receives 1+2+3+...+12, do you agree ?a, How many gifts are received on the the twelfth day?
Remember the formula from which Gauss gifted a sample to his teacher :
This is more tricky..b, How many gifts are received over the entire 12 days?
Let be the number of presents received on the n-th day.
From the formula above, .
We want to calculate
You already know what the second term is equal to..
Now, there is another formula you should know :
And normally, it's all done