Tr­êng­THCS­Nói­§Ìo­­­­­­­­­­­­­­N¨m­häc­2011 - 2012

Kiểm tra bài cũ:Kiểm tra bài cũ:

Nªu­c¸ch­tÝnh­diÖn­tÝch­tam­gi¸c­Nªu­c¸ch­tÝnh­diÖn­tÝch­tam­gi¸c­vµ­c¸ch­tÝnh­diÖn­tÝch­h×nh­ch÷­vµ­c¸ch­tÝnh­diÖn­tÝch­h×nh­ch÷­nhËt?­ViÕt­c«ng­thøc­tæng­qu¸t­nhËt?­ViÕt­c«ng­thøc­tæng­qu¸t­cña­chóng?cña­chóng?

1)­1)­C«ng­thøc­tÝnh­diÖn­tÝch­h×nh­thangC«ng­thøc­tÝnh­diÖn­tÝch­h×nh­thangH·y­chia­h×nh­thang­ABCD­(AB//CD)­thµnh­hai­tam­gi¸c­råi­tÝnh­diÖn­tÝch­h×nh­thang­theo­hai­®¸y­vµ­®­êng­cao.

?1

BA

D CH C

A K

= 12(AB­+­CD).AH

VËy­SABCD

1)­1)­C«ng­thøc­tÝnh­diÖn­tÝch­h×nh­thangC«ng­thøc­tÝnh­diÖn­tÝch­h×nh­thang?1§Þnh lÝ

DiÖn­tÝch­h×nh­thang­b»ng­nöa­tÝch­cña­tæng­hai­®¸y­víi­chiÒu­cao: S = ­12

(a­+­b).h

----------

-----------------------

b

a

h

Bµi30(SGK):

I

FE

D C

BA

K

HG

SABCD­=­SGHIK

SABCD­=­SABFIKE+­SDKE+­SCIFSGHIK­=­SABFIKE+­SAGE+­SBHF

SDKE­=­SAGE SCIF­=­SBHF

DKE­=­AGE CIF­=­BHF

1)­1)­C«ng­thøc­tÝnh­diÖn­tÝch­h×nh­thangC«ng­thøc­tÝnh­diÖn­tÝch­h×nh­thang2)­C«ng­thøc­tÝnh­diÖn­tÝch­h×nh­b×nh­hµnh?2 Dùa­vµo­c«ng­thøc­tÝnh­diÖn­tÝch­h×nh­

thang­®Ó­tÝnh­diÖn­tÝch­cña­h×nh­b×nh­hµnh

HD C

BA

= 12(AB­+­CD).AH SABCD = 1

2(CD­+­CD).AH

= 12.2CD.AH

= CD.AHVËy SABCD

1) 1) C«ng thøc tÝnh diÖn tÝch h×nh thangC«ng thøc tÝnh diÖn tÝch h×nh thang

?2§Þnh lÝ

DiÖn­tÝch­h×nh­b×nh­hµnh­b»ng­tÝch­cña­mét­c¹nh­víi­chiÒu­cao­t­¬ng­øng­víi­c¹nh­®ã: S = ­a.h

2) C«ng thøc tÝnh diÖn tÝch h×nh b×nh hµnh

--------------a

h

Bµi­27/sgk-125.

A B

D C F E

V×­sao­h×nh­ch÷­nhËt­ABCD­vµ­h×nh­b×nh­hµnh­ABEF­l¹i­cã­cïng­diÖn­tÝch?­Suy­ra­c¸ch­vÏ­mét­h×nh­ch÷­nhËt­cã­cïng­diÖn­tÝch­víi­mét­h×nh­b×nh­hµnh­cho­tr­íc.

SABCD­=­AB.­ADSABEF­­=­AB.­ADDo­®ã­SABCD­=­SABEF

1)1)C«ng­thøc­tÝnh­C«ng­thøc­tÝnh­diÖn­tÝch­h×nh­thangdiÖn­tÝch­h×nh­thang

S = ­12(a­+­b).h

----------

-----------------------

b

a

h

2)2)C«ng­thøc­tÝnh­diÖn­C«ng­thøc­tÝnh­diÖn­tÝch­h×nh­b×nh­hµnhtÝch­h×nh­b×nh­hµnh

-----------------------a

h

S = ­a.h

3)­VÝ­dôCho­h×nh­ch÷­nhËt­víi­hai­kÝch­th­íc­a,­b

a)­H·y­vÏ­mét­tam­gi¸c­cã­mét­c¹nh­b»ng­c¹nh­cña­h×nh­ch÷­nhËt­vµ­cã­diÖn­tÝch­b»ng­diÖn­tÝch­cña­h×nh­ch÷­nhËt­®ã

ab

- - - - - - - - - - - - ---- ---- - - - - d-----------------

2b

ab

- - - - - - - - - - - - ----

--------------------------------- 2a

b)­H·y­vÏ­mét­h×nh­b×nh­hµnh­cã­mét­c¹nh­b»ng­c¹nh­cña­h×nh­ch÷­nhËt­vµ­cã­diÖn­tÝch­b»ng­nöa­diÖn­tÝch­cña­h×nh­ch÷­nhËt­®ã

ab -----------------------------

ab

-------------------

1)1)C«ng­thøc­tÝnh­C«ng­thøc­tÝnh­diÖn­tÝch­h×nh­thangdiÖn­tÝch­h×nh­thang

S = ­12(a­+­b).h

----------

-----------------------

b

a

h

2)2)C«ng­thøc­tÝnh­diÖn­C«ng­thøc­tÝnh­diÖn­tÝch­h×nh­b×nh­hµnhtÝch­h×nh­b×nh­hµnh

-----------------------a

h

S = ­a.h

3)­VÝ­dô

Bµi­tËp26(SGK):TÝnh­diÖn­tÝch­m¶nh­®Êt­h×nh­thang­ABED­theo­c¸c­®é­dµi­®·­cho­trªn­h×nh­vµ­biÕt­diÖn­tÝch­h×nh­ch÷­nhËt­ABCD­lµ­828­m2

4)­VËn­dông

D C E

BA23

31

1)1)C«ng­thøc­tÝnh­C«ng­thøc­tÝnh­diÖn­tÝch­h×nh­thangdiÖn­tÝch­h×nh­thang

S = ­12(a­+­b).h

----------

-----------------------

b

a

h

2)2)C«ng­thøc­tÝnh­diÖn­C«ng­thøc­tÝnh­diÖn­tÝch­h×nh­b×nh­hµnhtÝch­h×nh­b×nh­hµnh

-----------------------a

h

S = ­a.h

3)­VÝ­dô4)­VËn­dông5)­H­íng­dÉn­vÒ­nhµ:+)­N¾m­v÷ng­c«ng­thøc­tÝnh­diÖn­tÝch­cña­h×nh­thang­vµ­h×nh­b×nh­hµnh.+)­T×m­hiÓu­mèi­quan­hÖ­gi÷a­c¸c­c«ng­thøc­tÝnh­diÖn­tÝch­cña­c¸c­h×nh­®·­häc.

+)­N¾m­v÷ng­c«ng­thøc­tÝnh­diÖn­tÝch­cña­h×nh­thang­vµ­h×nh­b×nh­hµnh.+)­T×m­hiÓu­mèi­quan­hÖ­gi÷a­c¸c­c«ng­thøc­tÝnh­diÖn­tÝch­cña­c¸c­h×nh­®·­häc.+)­Lµm­c¸c­bµi­tËp:­27,28,29,30,31(­SGK)

Th¶o luËn nhãmCho­h×nh­thang­ABCD­(AB//CD)­víi­I­lµ­trung­®iÓm­cña­BC,­E­lµ­giao­®iÓm­cña­AI­vµ­DC.­Chøng­minh­r»ng­SABCD­=­SADE

---------------------

BA

CD E

I

Chøng­minhTa­cã­ABI­=­ECI­(g.c.g)

Mµ­SABCD­=­SAICD+­SABISADE­=­SAICD+­SECI

=>­SABI­=­SECI

VËy­SABCD­=­SADE­H

Do­®ã:­SABCD­=­

1(AB+CD).AH2

C

BA

D H K

Bµi tËp 27(SGK)

SABCD­=­SABKH

SABCD­=­SABCH+­SADHSABKH­=­SABCH+­SBCK

SADH­=­SBCK

ADH­=­BCK