Cho h×nh ch÷ nhËt ABCD. Trªn ®­êng chÐo BD lÊy ®iÓm P, gäi M lµ ®iÓm ®èi xøng cña ®iÓm C qua P.

a)     Tø gi¸c AMDB lµ h×nh g×?

b)    Gäi E vµ F lÇn l­ît lµ h×nh chiÕu cña ®iÓm M lªn AB, AD. Chøng minh EF//AC vµ ba ®iÓm E, F, P th¼ng hµng.

b)     Do AM //BD nªn gãc OBA = gãc MAE (®ång vÞ)

Tam gi¸c AOB c©n ë O nªn gãc OBA = gãc OAB

Gäi I lµ giao ®iÓm 2 ®ư­êng chÐo cña h×nh ch÷ nhËt AEMF th× tam gi¸c AIE c©n ë I nªn gãc IAE = gãc IEA.

Tõ chøng minh trªn : cã gãc FEA = gãc OAB, do ®ã  EF//AC  (1)            

 MÆt kh¸c IP lµ ®ư­êng trung b×nh cña tam gi¸c MAC nªn IP // AC   (2)

Tõ (1) vµ (2) suy ra ba ®iÓm E, F, P th¼ng hµng.                                          
lần sau có bài gì cứ hỏi nha –  chamhocdethihsgtoan 27-04-16 12:39 PM
bài dễ mà bạn –  chamhocdethihsgtoan 27-04-16 12:39 PM
cảm ơn bạn –  hahahaha 27-04-16 12:31 PM

a)     Gäi O lµ giao ®iÓm 2 ®ư­êng chÐo cña h×nh ch÷ nhËt ABCD.

ð     PO lµ ®ư­êng trung b×nh cña tsm gi¸c CAM.

ð      AM//PO

 ð             tø gi¸c AMDB lµ h×nh thang.                                                                    

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