Ancient Greek mathematicians called 1, 3, 6, 10, 15, 21, ..., triangular numbers. According to its laws, what is the difference between the 8th triangular number and the 10th triangular number?
古希腊数学家把1，3，6，10，15，21，…，叫做三角形数，根据它的规律，则第8个三角形数与第10个三角形数的差是多少．
<gadget id="calculator">9 + 10</gadget>
<output>19</output>

<result>19</result>