|[http://www.textbooksonline.tn.nic.in/Books/Std09/Std09-II-SSS-EM-4.pdf Tamilnadu Transport and communication Page No 124-135] | |[http://www.textbooksonline.tn.nic.in/Books/Std09/Std09-II-SSS-EM-4.pdf Tamilnadu Transport and communication Page No 124-135] |