1:

(SAB), (SBC) vuông góc (BAC)

=>SB vuông góc (ABC)

AC vuông góc AB,SB

=>AC vuông góc (SAB)

=>AC vuông góc BH

mà SA vuông góc BH

nên BH vuông góc (SAC)

=>BH vuông góc SC

mà SC vuông góc BK

nên SC vuông góc (BHK)

c: (SH;(BHK))=góc SHK=(SA;BHK)

BC=BA/cos60=2a

SC=căn SB^2+BC^2=ăcn 5

SB^2=SK*SC

=>SK=a*căn 5/5

SA=căn SB^2+AB^2=a*căn 2

SB^2=SH*SA

=>SH=a*căn 2/2

sin SHK=căn 10/5

=>góc SHK=39 độ

Bài 2: 

a: =>(x+5)(4-x)=0

=>x=4 hoặc x=-5

b: =>2x(2x-1)=0

=>x=0 hoặc x=1/2

c: =>2x(x^2+1)+x^2+1=0

=>(x^2+1)(2x+1)=0

=>2x+1=0

=>x=-1/2

d: Δ=(-3)^2-4*1*4=9-16=-7<0

=>PTVN

1 weight

2 daily

3 endless

4 discussion 

5 available 

6 organizing 

7 happier

8 attractive

9 ashamed  

10 desicion 

b: \(BC=\sqrt{89}\left(cm\right)\)

\(\sin\widehat{B}=\dfrac{5\sqrt{89}}{89}\)

\(\Leftrightarrow\widehat{B}\simeq32^0\)

\(\widehat{C}=58^0\)

S=a(a+2)

1.Yes, they do

2..Yes, it is

3.People buy fruits and flowers from the market and decorate their house

4.People visit their family and friends