... Cho l a n g t r u A B C A ' B ' C c6 dai canh ben bang 2a, day ABC la t a m giac vuong t a i A, A B = a, AC = aVs cua dinh A va h i n h chieu vuong goc t r e n m a t p h a n g (ABC) la t r u ... mSt phang (a) Ta CO d (A, (a) ) = AK < AH (tinh chat dtfofng vuong goc va di/dng xien) Do khoang each tif A den (a) lorn nhat va chi AK = AH, hay K = H Suy (a) qua H va nhan AH = (1; - ; 1) lam vectcf ... : VOOAB = ^.BH.SAOO Ta c6: A ' B = A / A B " - A ' A " = aj3 BD = VA'D' -A' B' = ^ A B O ' D deu 3VD OHO =a B H = ^ V i AOO' la tarn giac vuong can canh ben bSng a n e n : SAOQ' = - ^a^ Vay the...