Ta có: \(AB^2\) = BH . BC  ;  \(AC^2\) = CH . BC

    Ta có:

    ⇒ BH  = 49 . 1 = 49

    ⇒ CH = 576 . 1 = 576

a) Ta có: \(\dfrac{BH}{HC}=\left(\dfrac{AB}{AC}\right)^2\)

\(\Leftrightarrow\dfrac{BH}{HC}=\dfrac{49}{576}\)

hay \(BH=\dfrac{49}{576}HC\)

Ta có: BH+HC=BC(H nằm giữa B và C)

\(\Leftrightarrow HC\cdot\dfrac{625}{576}=625\)

hay HC=576(cm)

\(\Leftrightarrow HB=BC-BH=625-576=49\left(cm\right)\)

27/12/2017 lúc 18:59

Ex1: Điền từ thích hợp vào chỗ trống

 This is Ba. He(1)......... a student.Every morning he(2).........up at 5.30.He(3).............. his teeth and takes a(4)............... then has breakfast at 6.15. He goes to school(5)........six thirty.His house is(6).............his house so he walks.The classes(7)............at 7.15 and finish at 11.15.In the afternoon he plays sports with his friend,Nam. They play badminton but now they(8).................soccer.In the evening he (9)......his homework and goes to(10).........at 9.30

Ex2:Cho dạng đúng của động từ trong ngoặc

1.My sister(have)...........classes from Monday to Friday

2.She(read)................a book in her room now

3.He(get)........................up at 6.00 every day?

4.There(not be)..............a big yard behind his classroom

27/12/2017 lúc 18:59

Ex1: Điền từ thích hợp vào chỗ trống

 This is Ba. He(1)......... a student.Every morning he(2).........up at 5.30.He(3).............. his teeth and takes a(4)............... then has breakfast at 6.15. He goes to school(5)........six thirty.His house is(6).............his house so he walks.The classes(7)............at 7.15 and finish at 11.15.In the afternoon he plays sports with his friend,Nam. They play badminton but now they(8).................soccer.In the evening he (9)......his homework and goes to(10).........at 9.30

Ex2:Cho dạng đúng của động từ trong ngoặc

1.My sister(have)...........classes from Monday to Friday

2.She(read)................a book in her room now

3.He(get)........................up at 6.00 every day?

4.There(not be)..............a big yard behind his classroom

27/12/2017 lúc 18:59

Ex1: Điền từ thích hợp vào chỗ trống

 This is Ba. He(1)......... a student.Every morning he(2).........up at 5.30.He(3).............. his teeth and takes a(4)............... then has breakfast at 6.15. He goes to school(5)........six thirty.His house is(6).............his house so he walks.The classes(7)............at 7.15 and finish at 11.15.In the afternoon he plays sports with his friend,Nam. They play badminton but now they(8).................soccer.In the evening he (9)......his homework and goes to(10).........at 9.30

Ex2:Cho dạng đúng của động từ trong ngoặc

1.My sister(have)...........classes from Monday to Friday

2.She(read)................a book in her room now

3.He(get)........................up at 6.00 every day?

4.There(not be)..............a big yard behind his classroom

Dễ quá đi

Ta có:

\(AB^2=BC\cdot BH=c^2=a\cdot c'\)

\(\Rightarrow c\cdot c=a\cdot c'\Rightarrow\dfrac{a}{c}=\dfrac{c}{c'}\)

Vậy đáp án đúng là D 

Ý cuối câu b.

Sử dụng công thức tính diện tích tam giác ABC. Ta có:

\(\frac{1}{2}AB.\sin\widehat{A}.AC=\frac{1}{2}AH.BC\)

=> \(AB.\sin\widehat{A}.AC=AH.BC\)

Ta đã tính được: \(AH=3\sqrt{3};AB=6;AC=2\sqrt{13};MN=\frac{18\sqrt{13}}{13};BC=8\) ( để tính MN sử dụng tam giác đồng dạng ở câu b ý 1 nha)

=> \(\sin\widehat{A}.AH=\frac{AH^2.BC}{AB.AC}=\frac{18\sqrt{13}}{13}=MN\)

tính MN sử dụng cặp tỉ số đồng dạng đúng không ạ ?

Đúng rồi em! 

\(\frac{AN}{AC}=\frac{MN}{BC}\), AN, AC, BC đều tính đc và từ đó suy ra MN.

a: Xét ΔABC vuông tại A có AH là đường cao

nên AB^2=BH*BC và AC^2=CH*BC

=>AB^2/AC^2=BH/CH

b: S AHC=8,64

=>1/2*AH*HC=8,64

=>AH*HC=17,28

S AHB=15,36

=>1/2*AH*HB=15,36

=>AH*HB=30,72

mà AH*HC=17,28

nên AH*AH*HB*HC=30,72*17,28

=>AH^2*AH^2=30,72*17,28

=>AH^4=530,8416

=>\(AH=\sqrt[4]{530.8416}=4.8\left(cm\right)\)