| For triangle numbers, imagine a set of ten bowling pins. It's a row of 1 pin + a row of 2 pins + a row of 3 pins + a row of 4 pins = 10 pins total. If you add another row, to retain the triangular shape, it would need to be 5 pins. Therefore, you would have 1+2+3+4+5 = 15 pins. It's kinda hard to explain but once you get it, it's easy enough. |