Gizatulla Imashev, Beket Z. Kenzegulov, Zhannat I. Sardarova, Valentina E. Makhatova, Gulmira K. Tashkeyeva, Samal B. Nugumanova, Bakytgul T. Abykanova
INT ELECT J MATH ED, Volume 11, Issue 7, 2016, 2163-2177