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nguyenbao01012004
2 năm trước
Cho hình choùp SABCD, coù ñaùy laø hình vuoâng taâm O. SA  (ABCD). Goïi H, I, K laàn löôït laø hình chieáu vuoâng goùc cuûa A treân SB, SC, SD. a) CMR: BC  (SAB), BD  (SAC). b) C/m: CD  SD. c) CMR: AH, AK cuøng vuoâng goùc vôùi SC. Töø ñoù suy ra 3 ñöôøng thaúng AH, AI, AK cuøng naèm trong moät maët phaúng.
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