1/6 + 4/21 + 1/56 + 9/136 + 8/425 + 12/925 = 35/74.

là 1 nha

1/6 + 4/21 + 1/56 + 9/136 + 8/425 + 12/925

= 1/2.3 + 4/3.7+ 1/7.8+9/8.17+1/17.25+12/25.37

=1/2-1/3+1/3-1/4+1/7-1/8+1/8-1/17+1/17-1/25+`1/25-1/37

=1/2-1/37

=35/74

(9+13+17+...+85)-(x+x+x+...+x)=925

bn tự làm tiếp nha

=( 9 + 13 +....+85 ) - ( x + x +.....+ x) =925

=940 - x.20 = 925

           x.20 = 940-925

           x.20 = 15

               x = 15:20

               x = 0.75

a)\(\left(5^{23}.5^{18}\right):5^{39}-5^0.0^5\)

= \(5^{41}:5^{39}-1.0\)

= \(5^2-0\)

= \(25-0=25\)

b)\(\left[2.3^3+144:\left(7^2-925:5^2\right)\right].\left|-8\right|\)

= \(\left[2.27+144:\left(49-925:25\right)\right].8\)

= \(\left[54+144:\left(49-37\right)\right].8\)

= \(\left[54+144:12\right].8\)

= \(\left[54+12\right].8\)

= \(66.8\)

= 528

1, \(\frac{5}{21}+0,5-\frac{19}{23}+\frac{16}{21}-\frac{4}{23}\)

\(=\frac{5+16}{21}-\frac{19+4}{23}+0,5\)

\(=1-1+0,5\)

\(=0,5\)

2,\(\frac{27}{23}+\frac{5}{21}-\frac{4}{23}+\frac{6}{21}+\frac{1}{2}\)

\(=\frac{27-4}{23}+\frac{5+6}{21}+\frac{1}{2}\)

\(=1+\frac{11}{21}+\frac{1}{2}\)

\(=\frac{42+22+21}{42}\)

\(=\frac{85}{42}\)

\(\dfrac{16}{21}+\dfrac{4}{23}+\dfrac{5}{21}-\dfrac{4}{23}-\dfrac{5}{21}\)

\(=\left(\dfrac{16}{21}+\dfrac{5}{21}-\dfrac{5}{21}\right)+\left(\dfrac{4}{23}-\dfrac{4}{23}\right)\)

\(=\dfrac{16}{21}+0+0\)

\(=\dfrac{16}{21}\)

\(\dfrac{16}{21}+\dfrac{4}{23}+\dfrac{5}{21}-\dfrac{4}{23}-\dfrac{5}{21}\)

\(=\left(\dfrac{16}{21}+\dfrac{5}{21}-\dfrac{5}{21}\right)+\left(\dfrac{4}{23}-\dfrac{4}{23}\right)\)

\(=\left(\dfrac{16+5-5}{21}\right)+\left(\dfrac{4-4}{23}\right)\)

\(=\dfrac{16}{21}+0\)

\(=\dfrac{16}{21}\)

- ( 12 + 21 - 23 ) - ( 23 - 21 + 10 ) 
= (- 10) - 12
= -22