20 x 9 : 2 = 180 : 2

= 90

81 : 9 + 10 = 9 + 10 = 19

20 x 9 : 2 = 180 : 2 = 90

81 : 9 + 10 = 9 + 10 = 19

20 x 9 : 2 = 180 : 2 = 90

81 : 9 + 10 = 9 + 10

= 19

20 x 9 : 2 = 180 : 2

= 90

a: Thay x=9 vào A, ta được:

\(A=\dfrac{3+2}{3-5}=\dfrac{5}{-2}=\dfrac{-5}{2}\)

\(B=\dfrac{3\sqrt{x}-15+20-2\sqrt{x}}{x-25}=\dfrac{\sqrt{x}+5}{x-25}=\dfrac{1}{\sqrt{x}-5}\)

b: Để \(A=B\cdot\left|x-4\right|\) thì \(\left|x-4\right|=\dfrac{A}{B}=\dfrac{\sqrt{x}+2}{\sqrt{x}-5}:\dfrac{1}{\sqrt{x}-5}=\sqrt{x}+2\)

\(\Leftrightarrow x-4=\sqrt{x}+2\)

\(\Leftrightarrow x-\sqrt{x}-6=0\)

=>x=9

   \(\dfrac{2\times9\times16\times25}{12\times20\times30}\)

=  \(\dfrac{2\times3\times3\times4\times4\times5\times5}{3\times4\times4\times5\times5\times3\times2}\)

= \(\dfrac{2\times3\times3\times4\times4\times5\times5}{2\times3\times3\times4\times4\times5\times5}\)

= 1

=360000

1 mới đúng

     A=1+9+9²+...+9²⁰

   9A=9+9²+9³⁺..+9²¹

9A-A=(9+9²+9³+...+9²¹)-(9⁰+9+9²+...+9²⁰)

    8A=9+9²+9³+...+9²¹-9⁰-9-9²-...-9²⁰

      8A=9²¹-9

       A=9²¹-9/8

A=(21-9)/8

ở trên mk ghi lộn

a) 172 + 10 x 2 = 170 +20 = 192

b) 10 x 2 + 300 = 20 + 300 = 320

c) 69 – 54 : 6 = 69 – 9 = 60

d) 900 + 9 x 10 = 900 +90 = 990

e) 20 x 6 + 70 = 120 + 70 = 190

f) 72 + 300 x 3 = 72 + 900 = 972