=1+1/2.(3.2/2)+1/3.(4.3/2)+1/4.(5.4/2)+...+1/20.(21.20/2)

=1+3/2+2+5/2+...+21/2 ( rút gọn)

=2/2+3/2+4/2+5/2+...+21/2

=(2+3+4+5+...+21)/2=(20.23)/2.2=(20.23)/4

=23.5=115

Ko bt mk lm dung ko nx?

\(1+\frac{1}{2}\left(1+2\right)+\frac{1}{3}\left(1+2+3\right)+...+\frac{1}{20}\left(1+2+3+...+20\right)=1+\frac{1}{2}.\frac{2.3}{2}+\frac{1}{3}.\frac{3.4}{2}+...+\frac{1}{20}.\frac{20.21}{2}=1+\frac{3}{2}+\frac{4}{2}+...+\frac{21}{2}=1+\frac{24.19}{2}=229\)

Ta có:

 B=1+1/2*(1+2)+1/3*(1+2+3)+..+1/20*(1+2+3+...+20)

B=1+3/2+6/3+10/4+...+210/20

=2/2+3/2+4/2+5/2+...+21/2=115

115 nhé bạn

mk học rồi nhưng quên mất

c) Ta có: \(\dfrac{3}{5}+\dfrac{-5}{20}+\dfrac{30}{75}+\dfrac{-7}{4}\)

\(=\dfrac{3}{5}+\dfrac{2}{5}+\dfrac{-1}{4}+\dfrac{-7}{4}\)

\(=1-2=-1\)

Giải:

a)-1/12+4/3=-1/12+16/12=15/12=5/4

b)(-4/14-3/15)-(1/5-20/35-(-1)).7

=-17/35-22/35.7

=-17/35-22/5

=-171/35

c)3/5+-5/20+30/75+-7/4

=3/5+-1/4+2/5+-7/4

=(3/5+2/5)+(-1/4+-7/4)

=1+-2

=-1

d)5/6.-12/14+7/13

=-5/7+7/13

=-16/91

e)2/-9-5/-36-1/4

=-1/12-1/4

=-1/3

f)2/23+-5/12+7/18+21/23+-7/12

=(2/23+21/23)+(-5/12+-7/12)+7/18

=1+-1+7/18

=7/18

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1207​:2,7+2,7:1,35+(0,4:212221​).(4,2-13401403​^₎=

   \(1\dfrac{7}{20}\div2,7+2,7\div1,35+\left(0,4\div2\dfrac{1}{2}\right)\times\left(4,2-1\dfrac{3}{40}\right)\)

\(=\dfrac{27}{20}\div\dfrac{27}{10}+\dfrac{27}{10}\div\dfrac{27}{20}+\left(\dfrac{2}{5}\div\dfrac{5}{2}\right)\times\left(\dfrac{21}{5}-\dfrac{43}{40}\right)\)

\(=\dfrac{27}{20}\times\dfrac{10}{27}+\dfrac{27}{10}\times\dfrac{20}{27}+\left(\dfrac{2}{5}\times\dfrac{2}{5}\right)\times\dfrac{25}{8}\)

\(=\dfrac{1}{2}+2+\dfrac{4}{25}\times\dfrac{25}{8}\)

\(=\dfrac{5}{2}+\dfrac{1}{2}\)

\(=3\)