you can download now Grade 11 local syllabus Mathematics Sinhala medium school textbooks for free.
click the following link and download.
1st part
http://www.edupub.gov.lk/SelectChapter.php
2nd part
http://www.edupub.gov.lk/SelectChapter.php
3rd part
http://www.edupub.gov.lk/SelectChapter.php