Câu 9: A

Câu 10: C

Câu 11: C

Câu 12: A

Câu 13; B

Câu 14: C

 A

 C

C

 A

 B

C

25B

24B

23B

21A
22C

`7/12 + 3/4 xx 2/9`

`= 7/12 + 1/6`

`= 9/12`

_________________

`8/9 - 4/15 : 2/5`

`= 8/9 - 2/3`

`= 2/9`

giúp me 

a) _{^{\(\dfrac{7}{12}+\dfrac{3}{4}\times\dfrac{2}{9}=\dfrac{7}{12}+\dfrac{1}{6}=\dfrac{7}{12}+\dfrac{2}{12}=\dfrac{3}{4}\)}}

b) _{^{\(\dfrac{8}{9}-\dfrac{4}{15}:\dfrac{2}{5}=\dfrac{8}{9}-\dfrac{4}{15}\times\dfrac{5}{2}=\dfrac{8}{9}-\dfrac{2}{3}=\dfrac{8}{9}-\dfrac{6}{9}=\dfrac{2}{9}\)}}

ta có :              

16 = 2⁴          72 = 2³. 3²                     ƯCLN ( 16 , 72 ) = 2³ = 8      vậy ƯC ( 16 , 72 ) = 8

( 7 - \(x\))³ + (11 - 7)² = 141

(7 - \(x\))³ + 4² = 141

( 7 - \(x\))³ + 16  = 141

(7 - \(x\))³          = 141 - 16

( 7 - \(x\))³        = 125

 (7 - \(x\))³       = 5³

 7 - \(x\)          = 5

       \(x\)         = 7 - 5

       \(x\)          = 2

\(\left(7-x\right)^3+\left(11-7\right)^2=141\)

\(\left(7-x\right)^3+4^2=141\)

\(\left(7-x\right)^3+16=141\)

\(\left(7-x\right)^3=141-16\)

\(\left(7-x\right)^3=125\)

\(\left(7-x\right)=5^3\)

\(\Rightarrow7-x=5\)

\(x=7-5\)

\(x=2\)

\(\text{Vậy x=2}\) 

( 7 - $x$)3 + (11 - 7)2 = 141

(7 - $x$)3 + 42 = 141

( 7 - $x$)3 + 16  = 141

(7 - $x$)3          = 141 - 16

( 7 - $x$)3        = 125

 (7 - $x$)3       = 53

 7 - $x$          = 5

       $x$         = 7 - 5

       $x$          = 2

2 phần 3 j z bạn 

\(\alpha\in\left(-90;0\right)\Rightarrow\left\{{}\begin{matrix}sina< 0\\cosa>0\end{matrix}\right.\) \(\Rightarrow cosa=\sqrt{1-sin^2a}=\dfrac{3}{5}\)

\(cot\left(a+60^0\right)=\dfrac{cos\left(a+60^0\right)}{sin\left(a+60^0\right)}=\dfrac{cosa.cos60^0-sina.sin60^0}{sina.cos60^0+cosa.sin60^0}\)

\(=\dfrac{\dfrac{3}{5}.\dfrac{1}{2}-\left(-\dfrac{4}{5}\right).\dfrac{\sqrt{3}}{2}}{-\dfrac{4}{5}.\dfrac{1}{2}+\dfrac{3}{5}.\dfrac{\sqrt{3}}{2}}=...\)

\(sin\left(45^0-a\right)=sin45^0.cosa-cos45^0.sina=\dfrac{\sqrt{2}}{2}.\dfrac{3}{5}-\dfrac{\sqrt{2}}{2}.\left(-\dfrac{4}{5}\right)=...\)

Bài 5:

a: 2x-(3-5x)=4(x+3)

=>2x-3+5x=4x+12

=>7x-3=4x+12

=>3x=15

=>x=5

b: =>5/3x-2/3+x=1+5/2-3/2x

=>25/6x=25/6

=>x=1

c: 3x-2=2x-3

=>3x-2x=-3+2

=>x=-1

d: =>2u+27=4u+27

=>u=0

e: =>5-x+6=12-8x

=>-x+11=12-8x

=>7x=1

=>x=1/7

f: =>-90+12x=-45+6x

=>12x-90=6x-45

=>6x-45=0

=>x=9/2

14/9