thêm đề để luyện phần chương 2 và 3

http://www.onthi.com/store/download/phuong_phap_dung_gian_do_vec_to_giai_bai_tap_dien_xoay_chieu_5812_73836711.pdf

http://www.onthi.com/store/download/Bai_tap_trac_nghiem_ly_4518_88375703.pdf
http://vatly.net/khotainguyentracnghiem.php?115%20bai%20%20phan%20song.1415.pdf
http://thuvienvatly.com/home/index.php?option=com_remository&func=startdown&id=1938
http://vatly.net/khotainguyentracnghiem.php?115%20bai%20%20phan%20song.1415.pdf