\(\dfrac{5}{13}+\dfrac{-5}{17}+\dfrac{-20}{41}+\dfrac{8}{13}+\dfrac{-21}{41}\)

\(=\left(\dfrac{5}{13}+\dfrac{8}{13}\right)+\dfrac{-5}{17}+\left(\dfrac{-20}{41}-\dfrac{21}{41}\right)\)

\(=\dfrac{13}{13}+\dfrac{-5}{17}+\dfrac{-41}{41}\)

\(=1-\dfrac{5}{17}-1\)

\(=-\dfrac{5}{17}\)

Ta có: \(A=\dfrac{5}{13}+\dfrac{-5}{7}+\dfrac{-20}{41}+\dfrac{8}{13}+\dfrac{-21}{41}\)

\(=\left(\dfrac{5}{13}+\dfrac{8}{13}\right)+\left(\dfrac{-20}{41}+\dfrac{-21}{41}\right)+\dfrac{-5}{7}\)

\(=1-1+\dfrac{-5}{7}\)

\(=\dfrac{-5}{7}\)

Có 2 dấu = hả bạn

\(A=\dfrac{5}{13}+\dfrac{-5}{7}+\dfrac{-20}{41}+\dfrac{8}{13}+\dfrac{-21}{41}=(\dfrac{5}{13}+\dfrac{8}{13})+(\dfrac{-20}{41}+\dfrac{-21}{41})+(\dfrac{-5}{7})=1+\left(-1\right)+\dfrac{-5}{7}=0+\dfrac{-5}{7}=\dfrac{-5}{7}\)

\(\dfrac{5}{13}+\dfrac{5}{17}+\dfrac{-20}{41}+\dfrac{8}{13}+\dfrac{-21}{41}\)

\(=\left(\dfrac{5}{13}+\dfrac{8}{13}\right)+\left(\dfrac{-20}{41}+\dfrac{-21}{41}\right)+\dfrac{5}{17}\)

\(=1+\left(-1\right)+\dfrac{5}{17}\)

\(=0+\dfrac{5}{17}=\dfrac{5}{7}\)

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\(=\left(\dfrac{5}{13}+\dfrac{8}{13}\right)+\left(\dfrac{-20}{41}+\dfrac{-21}{41}\right)+\dfrac{5}{17}\)

\(=1+\left(-1\right)+\dfrac{5}{17}\)

\(=0+\dfrac{5}{17}\)

\(=\dfrac{5}{17}\)

\(\dfrac{5}{13}+\dfrac{5}{17}+\dfrac{-20}{41}+\dfrac{8}{13}+\dfrac{-21}{41}\)

= \(\left(\dfrac{5}{13}+\dfrac{8}{13}\right)+\left(\dfrac{-20}{41}+\dfrac{-21}{41}\right)+\dfrac{5}{17}\)

= \(1+\left(-1\right)+\dfrac{5}{17}\)

= \(0+\dfrac{5}{17}\)

= \(\dfrac{5}{17}\)

1+5+9+13+17+21+25

=1+(5+25)+(13+17)+(9+21)

=1+30+30+30

=1+30x3

=1+90

=91

chắc chắn 100% k cho mình nhata phuong anh

1+5+9+13+17+21+25=91  nha bn tk mk nha

91 nha nhóm các số vào

\(\frac{5}{13}+\left(-\frac{5}{7}\right)+\left(-\frac{20}{41}\right)+\frac{8}{13}+\left(-\frac{21}{41}\right)\)

\(=\left(\frac{5}{13}+\frac{8}{13}\right)+\left(-\frac{20}{41}+\frac{-21}{41}\right)+\left(-\frac{5}{7}\right)\)

\(=1+\left(-1\right)+\left(-\frac{5}{7}\right)\)

\(=0+\left(-\frac{5}{7}\right)=-\frac{5}{7}\)

\(\left(\frac{5}{13}+\frac{8}{13}\right)+\left(\frac{-20}{41}+\frac{-21}{41}\right)+\frac{-5}{7}\)

\(=1+\left(-1\right)+\frac{-5}{7}\)

\(=\left(1-1\right)+\frac{-5}{7}\)

\(=0+\frac{-5}{7}\)

\(=\frac{-5}{7}\)

Bài làm :

   1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20

= (1+19)+(2+18)+(3+17)+(4+16)+(5+15)+(6+14)+(7+13)+(8+12)+(9+11)+(10+20)

=   20   +   20   +   20   +   20  +   20    +   20  +   20   +    20   +   20  +30

=  20x9 +30

=180+30

=210

tk cho mik nhé

\(\frac{\left(20+1\right)x20}{2}\)

=(1+19)+(2+18)+(3+17)+(4+16)+(5+15)+(6+14)+(7+13)+(8+12)+(9+11)+10

=20+20+20+20+20+20+20+20+20+10

=190