Write two "+" signs in any two positions in the middle of the row of numbers below, and you can get the addition formula of adding 3 natural numbers. How many different addition formulas can you get in total. 1,2,3,4,5 ,6,7,8,9.
在下面一排数字中间的任意两个位置写上两个"+"号，可以得到3个自然数相加的加法算式，一共可以得到多少个不同的加法算式.1,2,3,4,5,6,7,8,9．
<gadget id="calculator">8 * 7</gadget>
<output>56</output>

<gadget id="calculator">2 * 1</gadget>
<output>2</output>

<gadget id="calculator">56 / 2</gadget>
<output>28</output>

<result>28</result>