9. (12 points) Shuaishuai memorized words for 7 days. Starting from the 2nd day, he memorized 1 more word each day than the previous day, and the sum of the number of words memorized in the first 4 days is equal to the sum of the number of words memorized in the last 3 days. How many words did Shuaishuai memorize in total over these 7 days? $\qquad$
【Solution】Solution: According to the problem, we have:
Let the number of words Shuai Shuai memorizes be: $a, a+1, a+2, a+3, a+4, a+5, a+6$ for a total of 7 days
$$
a+a+1+a+2+a+3=a+4+a+5+a+6
$$

Solution: $a=9$.
Total words memorized: $9+10+11+12+13+14+15=84$
Therefore, the answer is: 84