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CHUYN T DIN V CC BI TON LIN QUANI TH TCH T DIN:Gi cc on thng c di nh hnh bn.
Dng cc on thng v
K
Ta c:
Suy ra:
Theo t s th tch ta c :
Gi , ,,
Do
Ta c:
S dng nh l Thals trong v vi ta c :
K nn ta c :
Li c : T y ta c :
Th cc i lng tnh ta c :
Vy ta c mi quan h gia gc 2 mt phng v gc nh l
Vy
Suy ra Mt khc :
T y ta c cng thc tnh th tch t din bt k :
Hay : T ta thy rng t din vung l t din c th tch ln nht trong cc t din.ng cao t din :
By gi ta tm :
Dng , suy ra
Vy
Nhng do l hnh bnh hnh nn
M
Gi MN l on vung gc chung cua AC v BDTa c:
Gi H l chn ng cao h t D xung mt phng Ta c: NMHD l hnh ch nht
Chng minh: Ta c: do MN v DH l khong cch gia hai ng thng cho nhau AC v BD nn NM // DH. Mt khc nn suy ra pcmM DN // CE nn suy ra MH // CE.
Trong tam gic ACE k MI // CE, Ta c :
K By gi ta s tnh di on TE: (Vi Tl hnh chiu ca A xung mp (BCD) ):
Ta c:
Tng t ta c:
Gi ta s thit lp cng thc tnh di mt on thng TE:
Ta c:
Suy ra:
Vi