a, \(\frac{9}{25}=\frac{36}{100}=36\%\) phân số này không có hỗn số 

b, \(\frac{17}{4}=4\frac{1}{4}=\frac{425}{100}=425\%\)

c,Rút gọn \(\frac{39}{65}=\frac{3}{5}\);   \(\frac{3}{5}=\frac{6}{10}=60\%\) phân sô này không có hỗn số 

 S=1/31+1/32+...+1/60

=> S=(1/31+1/32+...+1/40)+(1/41+1/42+...+1/50)+(1/51+1/52+...+1/60)

ta có 1/31<1/30, 1/32<1/30, ...,1/40<1/30

=> (1/31+1/32+...+1/40)< 1/30+1/30+..+1/30=> (1/31+1/32+...+1/40)< 10*1/30=> (1/31+1/32+...+1/40)<1/3

1/41<1/40, 1/42<1/40,..., 1/50<1/40

=> (1/41+1/42+...+1/50)< 1/40+1/40+..+1/40=> (1/41+1/42+...+1/50)<10*1/40=> (1/41+1/42+...+1/50)<1/4

1/51<1/50, 1/52<1/50,..< 1/60<1/50

=> (1/51+1/52+...+1/60)<1/50+1/50+..+1/50=> (1/51+1/52+..+1/60)<10*1/50=>(1/51+1/52+...+1/60)<1/5

=> S< 1/3+1/4+1/5=> S<47/60

vì 47/60<48/60=> 47/60<4/5=> S<4/5

Đổi: 30 phút = \(\frac{1}{2}\)giờ

Sau khi chảy trong 30 phút thì được: \(\frac{1}{2}:3=\frac{1}{2}.\frac{1}{3}=\frac{1}{6}\)( bể )

Đáp số: \(\frac{1}{6}\)phần của bể 

\(1.2.3...9-1.2.3...8-1.2.3...8^2\)

\(=\left(1.2.3...8\right).9-1.2.3...8-1.2.3..8.8\)

\(=\left(1.2.3....8\right)8-1.2.3.4...8\)

\(=1.2.3...8.8-1.2.3...8.8\)

\(=0\)

\(a)\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{132}\)

\(=\frac{22}{132}+\frac{11}{132}+\frac{1}{20}+\frac{1}{132}\)

\(=\frac{33}{132}+\frac{1}{20}+\frac{1}{132}\)

\(=\frac{34}{132}+\frac{1}{20}\)

\(=\frac{17}{66}+\frac{1}{20}\)

\(=\frac{203}{660}\)

\(a,\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{132}\) 

\(=\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{132}\)

\(=\left(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}\right)+\frac{1}{132}\)

\(=\left(\frac{1}{2}-\frac{1}{5}\right)+\frac{1}{132}\)

\(=\frac{3}{10}+\frac{1}{132}\)

\(=\frac{198}{660}+\frac{5}{660}\)

\(=\frac{203}{660}\)