BIÕN ®ÆI T NH− SAU
2) BiÕn ®æi T nh− sau :
2T = 1 − cos2A + 1 − cos2B + 1 − cos2C = 3 − [coss 2A + cos2B] − cos 2C =
= 3 − 2cos(A + B) cos (A − B) − cos 2C =
3 2cosCcos(A B) [2cos C 1]
+
−
−
2
− =
= 4 + 2cosC[cos (A − B) − cosC ] = 4 + 2cosC[cos (A − B) + cos(A + B)] =
= 4[1 + cosAcosBcosC].
VËy T = 2 + 2cosA cosB cosC.
Tõ ®ã thÊy r»ng T > 2 ⇔ ∆ ABC nhän, T = 2 ⇔∆ ABC vu«ng vµ T < 2 ⇔ ∆ ABC tï.
LuyÖn thi trªn m¹ng
_____________________________________________________
__________C©u
IVạ
1)
§
û
êng trßn (C) cã t©m C
(a ; m 2