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\(a)\frac{-3}{4}+\frac{3}{7}+-\frac{1}{4}+\frac{4}{9}+\frac{4}{7}\)
\(=\left(\frac{-3}{4}+-\frac{1}{4}\right)+\left(\frac{3}{7}+\frac{4}{7}\right)+\frac{4}{9}\)
\(=-1+1+\frac{4}{9}\)
\(=\frac{4}{9}\)
\(\frac{-3}{4}+\frac{3}{7}+\frac{-1}{4}+\frac{4}{9}+\frac{4}{7}\)
\(=\left(\frac{-3}{4}+\frac{-1}{4}\right)+\left(\frac{3}{7}+\frac{4}{7}\right)+\frac{4}{9}\)
\(=\left(-1\right)+1+\frac{4}{9}\)
\(=0+\frac{4}{9}\)
\(=\frac{4}{9}\)
\(A=\frac{21}{31}+\frac{-16}{7}+\frac{44}{53}+\frac{10}{21}+\frac{9}{53} \)
\(A=\left(\frac{16}{7}+\frac{10}{21}\right)+\left(\frac{44}{53}+\frac{9}{53}\right)+\frac{21}{31}\)
\(A=\frac{58}{21}+1+\frac{21}{31}\)
\(A=\frac{100}{21}\)
\(B=6\left(\frac{1}{15.18}+\frac{1}{18.21}+...+\frac{1}{87.90}\right)\)
\(B=6\left(\frac{1}{15}-\frac{1}{18}+\frac{1}{18}-\frac{1}{21}+...+\frac{1}{87}-\frac{1}{90}\right)\)
\(B=6\left(\frac{1}{15}-\frac{1}{90}\right)\)
\(B=6.\frac{1}{18}\)
\(B=\frac{1}{3}\)
\(\left(\frac{21}{31}-\frac{10}{31}\right)-\left(\frac{44}{53}+\frac{9}{53}\right)+\frac{1}{3}\)
\(\frac{11}{31}-1+\frac{1}{3}\)
\(\frac{-29}{93}\)
= 21/31 + 1/3 - 44/53 + 10/31 -9/53
= [21/31+10/31] - [44/53 + 9/53] +1/3
= 1-1+1/3 = 1/3
a)(1/4+3/4)-(5/13+8/13)+2/11=1-1+2/11=2/11
b)(21/31+10/31)-16/7+(44/53+9/53)=1+1-16/7=14/7-16/7=-2/7
1/2+2/3+3/4+4/5+5/6+6/7+7/8+8/9+9/10x9/10
=9/10x(1/2+2/3)+(3/4+4/5)+(5/6+6/7)+(7/8+8/9)
=9/10x(1/3+3/5+5/7+7/9)
9/10x(1/3+3/5)+(5/7+7/9)
=9/10x1/5+5/9
9/50+5/9
=10
Bn Long làm đúng rồi bn 
nguyễn kim arica cứ làm theo cách đó là được .
Bn nào thấy đúng thì ủng hộ nha .
\(\left(\frac{-3}{42}+\frac{10}{-21}-\frac{-9}{14}\right).\left(\frac{-7}{5}\right).\)
\(=\left(\frac{-1}{14}-\frac{10}{21}+\frac{9}{14}\right).\left(\frac{-7}{5}\right)\)
\(\left(\frac{12}{21}-\frac{10}{21}\right).\left(\frac{-7}{5}\right)=\frac{2.\left(-7\right)}{21.5}=\frac{-2}{15}\)
=21/31+10/31+
= 1 nhé