đề chọ hs giỏi toán 6

21x 2 + 5x x 3 + x 2 3.  − 2 3  3 − 2.  2 3  2 + 4.  − 2 3  + 1  1 2013 + 13 2014 − 5 2017  .  1 3 + 1 6 − 1 2  A B A = 5 17 + 1 5 19 + 2 B = 5 20 + 1 5 22 + 2 A = 10 15 + 5 10 15 − 7 B = 10 16 + 7 10 16 − 5 3 500 7 300 8 5 3.4 7 1990 10 + 1990 9 1991 10 xx  O xx  oy oz  xOz = 30 0  x  Oz = 4.  xOz Ox, Oy, Oz Oz xOy Oz  x  Oy  zOz  =?