[填空题]
The fifth term, $u_{5}$ , of an arithmetic sequence is 25 . The eleventh term, $u_{11}$ , of the same sequence is 49 .
1. Find d , the common difference of the sequence.
2. Find $u_{1}$ , the first term of the sequence.
3. Find $S_{100}$ , the sum of the first 100 terms of the sequence.
