... 21 = (1) B (S) ú -2a + 6b + d + 11 = (2) 211 C (S) ú -4a 4b + d + 17 = (3) (1) , (2), (3) cho a = -2, b = 1, d = - 21 =>(S): x2 + y2 + z2 + 4x 2y - 21 = Bi 15 : Cho im A(2,0 ,1) , B( -1, 1,3), C (1, 2,0) ... BC = ỡ- x + 11 y + z - = ỡ x = 22 ùù ù ù ú BH AC = ớy - = ớy = ù ùx + z - = ù z = - 21 ợ ợ ùợ AB, AC AH = [ ] Vy: H(22,4,- 21) Bi 10 : Cho im: A (1, 0 ,1) , B( -1, 1,2), C( -1, 1,0), D(2, -1, -2) a) Chng ... ti A ú = (-2, b, c -1) , = (-3 ,1, 2), (1) = ( -1, 2, -1) Do ú: ú (1) ú => I(0,-2, -1) Bỏn kớnh (S): R = IA = => (S): x2 + (y+2)2 + (z +1) 2 = 12 Bi 16 : Vit phng trỡnh mt cu (S) cú tõm I (1, -3,6) v ct trc...