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The iPeriscope
Required: 2 iPhones. 2 sticks and blue tack. A prodding device. Operate by taking photo with upper device. Email to lower device. View.
Sum of integers
This is why the sum of n integers is 1/2n(n+1). Someone called Gauss worked it out in his head at school, but I like this picture. If you make a rectangle n wide and n+1 high, and shade half of … Continue reading