` a,` 

`1/3 + 3/4 xx 5/9 `

`=1/3 + (3/4 xx 5/9) `

`= 1/3 + 5/12 `

`= 4/12 + 5/12`

`= 9/12 `

`= 3/4 .`

`b,`

` 7/12 - 5/4 xx 2/15 `

`= 7/12 - (5/4 xx 2/15)`

`= 7/12 - 1/6`

`= 7/12 - 2/12 `

`= 5/12 .`

a: \(A=\left(2x-1\right)\left(4x^2+2x+1\right)-7\left(x^3+1\right)\)

\(=\left(2x\right)^3-1^3-7x^3-7\)

\(=8x^3-1-7x^3-7=x^3-8\)

b: Thay x=-1/2 vào A, ta được:

\(A=\left(-\dfrac{1}{2}\right)^3-8=-\dfrac{1}{8}-8=-\dfrac{65}{8}\)

c: \(A=x^3-8=\left(x-2\right)\left(x^2+2x+4\right)\)

Để A là số nguyên tố thì x-2=1

=>x=3

\(=8x^3+27-8x^3+2+8=37\left(đpcm\right)\)

=8x^3+27_8x^3+2+8

=37

\(\dfrac{7}{9}+\left(\dfrac{2}{3}-\dfrac{5}{12}\right)\)
\(=\dfrac{7}{9}+\left(\dfrac{8}{12}-\dfrac{5}{12}\right)\)
\(=\dfrac{7}{9}+\dfrac{1}{4}\)
\(=\dfrac{28}{36}+\dfrac{9}{36}=\dfrac{37}{36}\)
\(-------------\)
\(\dfrac{5}{8}+\dfrac{1}{4}\times\dfrac{2}{3}\)
\(=\dfrac{5}{8}+\dfrac{1}{6}\)
\(=\dfrac{15}{24}+\dfrac{4}{24}=\dfrac{19}{24}\)

\(\dfrac{7}{9}+\left(\dfrac{2}{3}-\dfrac{5}{12}\right)=\dfrac{7}{9}+\dfrac{1}{4}=\dfrac{28}{36}+\dfrac{9}{36}=\dfrac{37}{36}\)

_____________________________________

\(\dfrac{5}{8}+\dfrac{1}{4}\times\dfrac{2}{3}=\dfrac{5}{8}+\dfrac{1}{6}=\dfrac{15}{24}+\dfrac{4}{24}=\dfrac{19}{24}\)

a, 654 – 10 x 4                                       b, 16 x 3 : 8

  =  654 - 40       (0,5đ)                              =    48 : 8      (0,5đ)

  =      614          (0,25đ)                            =      6          (0,25đ)

a, 754 – 10 x 4                                      b, 16 x 3 : 8

     =  754 - 40       (0,5đ)                           =    48 : 8      (0,5đ)

     =      714          (0,25đ)                         =      6          (0,25đ)