Rec = r = loọ DoR,6, = {(4f i* : loc)
R, +R, +R3
n6n ta tim dugc R: = l5C).
Cudng dQ đng di0n qua dbn Dr vh Dz lAn
lucrt li ^
DAE + sin ADE
sin2aln ln IS l. =slna /^ t ( bae+aon DAE-ADE\ --l -.cos- | cosat --"--\ 2 - 2 /l ( fri-;rr ) =cosl ":-"--::::- 131. [2) lll hu hl dsina
Ding thuc xay ra khi vh chi khi frE =m
Q EA = ED,hay BA = BD vd. CA = CD. A (Nn4" x6t. l) K6t qua bii todn v6n tiling v6i moi tu
<liQn ABCD th6a mdn rli6u kien BC L AD. TAt ca c6c loi
girii gLri v€ Tda so4n c16u dring vd tuong tU nhu ldi gitii
tr6n. 'luy nhiOn, nhi6u b4n qu€n chi ra khi nho thi d6ng
th[rc xaiy ra, MOt s0 ban su dyng cOng tht'c tinh diQn
tich rnQt phdttg ph4n gidc nhi diQn cqnh B.C cila t* diQn ARCD (trong Tuy€n chpn theo chuyAn d€ Todn h7c vit 'l'tt6i trd, Quy€n 4, trang l6l), cfing cho loi gitii ggn. 2) Ngoiii ban Qtfin,c5c ban sau cfrng c6 loi girii t6t:
ÍIi NQi: Ngul,in Minh Tl'tudtt, I 1 Toiin l, truong THPT chuy0n DHSP He NQi; Vinh Phirc: Nguy€n Minh
'fhudn,, I lAl0, THPT chuyOn Vinh Phric, Hodng Vfi
Hi€p, I lAl, THPT Ng6 Gia Tw, L0p Thach; Hiii
Durrng: Nguy€n Vdn Dfing, I lAl, THP'I Ke Sat, Binh
Giang; H:ii Phbng: Phan Dwc Minh, llAl5, THPT 'l'hdi Phi6n; Thanh H6a: La Hing Qudn,l lT, TFIPT
chuyOn I-am- Son; Ngh$ An: Nguy€n Vdn Hodng,
10'11 , Ngut€n Duy Hai,llA7, THPT DO l-uong I,
DQng.Hfr'u Treng, 11,41, THPT Quj, HSp 2, Llo_dng
Viil Anh, IlAl, THPT Thanh Chuong 3; Dir Nfrng:
Bii Ddng Khoa, l lAl, THPT chuyOn LC Quf DOn;
Qu:ing Nam: Nguy€n H6ng Sctn, 1211, THPT chuyOn
Nguy0n Binh Khi0ml Binh Dinh: LA Nhtr Nggc, l0T,
'trdn [)oitn Tdn, l1T, THPT chuy0n L0 Quj DOu
Kh6nh Hda Trin o6 ru*u Todn, 11 To6n, THPT
chuy0n L0 Qu! DOn, TP. Nha Trang; Phr6YGn: Phan Minh Tri, ll Toiin, THPT chuyOn Luong V6n Ch6nh, TP. Tuy Hda, 7d Vdn Huy, llAl, TIIPT LO H6ng
Phong, T6y Hda; Cin Tho: L€ D(ti Thdnh, 12A1, THP'Í chuy6n Lll Tu Trgng; B6n \-re: Kh6ng Htiu
lliQp, I lT, TÍIPT chuyOn BOn Trẹ
Hd euANG vrN[r
* Ilii l,L/394. Clut ntctch di)n nltu ltinh vd.
l)iítr tt'tt r'= I(X), t',ir'tlítr Dt vi l). t r't cfir;,, ,r,,'