8 5/7+4/21 = 8 15/21+4/21 = 8 19/21 см - длина NK 8 5/7+8 19/21 = 8 15/21+8 19/21 = 16 34/21 = 17 13/21 см - длина MN и NK 30-17 13/21 = 29 21/21-17 13/21 = 12 8/21 - длина MK