D

D

B

B

\(B=1+\frac{1}{2}\left(1+2\right)+\frac{1}{3}\left(1+2+3\right)+...+\frac{1}{12}\left(1+2+3+...+12\right)\)

\(B=1.\left(\frac{1.2}{2}\right)+\frac{1}{2}.\left(\frac{2.3}{2}\right)+\frac{1}{3}.\left(\frac{3.4}{2}\right)+....+\frac{1}{12}.\left(\frac{12.13}{2}\right)\)

\(B=\frac{2}{2}+\frac{3}{2}+\frac{4}{2}+....+\frac{13}{2}=\frac{2+3+4+....+13}{2}=\frac{90}{2}=45\)

ban lam sai rui de mk lam lai nhe.

\(12.\left(x-1\right):3=4^3-2^3\)

\(12.\left(x-1\right):3=64-8\)

\(12.\left(x-1\right):3=56\)

\(12.\left(x-1\right)=56.3\)

\(12.\left(x-1\right)=168\)

\(x-1=168:12\)

\(x-1=14\)

             \(x=15\)

kkkkkkkkkkkkkkkkkkkkkkkkkkkkk

   \(\dfrac{2}{5}+\dfrac{2}{3}+\dfrac{2}{4}\)

= \(\dfrac{24}{60}\) + \(\dfrac{40}{60}\) + \(\dfrac{30}{60}\)

= \(\dfrac{64}{60}\) + \(\dfrac{30}{60}\)

= \(\dfrac{47}{30}\)

\(\dfrac{2}{6}+\dfrac{3}{12}\)

= \(\dfrac{4}{12}\) + \(\dfrac{3}{12}\)

= \(\dfrac{7}{12}\)

\(\dfrac{5}{6}\) + \(\dfrac{1}{3}\)

= \(\dfrac{5}{6}\) + \(\dfrac{2}{6}\)

= \(\dfrac{7}{6}\)

   \(\dfrac{1}{3}\) + \(\dfrac{5}{12}\) + \(\dfrac{5}{6}\)

= \(\dfrac{4}{12}\) + \(\dfrac{5}{12}\) + \(\dfrac{10}{12}\)

= \(\dfrac{9}{12}\) + \(\dfrac{10}{12}\)

= \(\dfrac{19}{12}\)

    \(\dfrac{5}{8}\) + \(\dfrac{4}{7}\)

= \(\dfrac{35}{56}\) + \(\dfrac{32}{56}\)

=  \(\dfrac{67}{56}\) 

\(\dfrac{7}{3}\) + \(\dfrac{8}{7}\)

= \(\dfrac{49}{21}\) + \(\dfrac{24}{21}\)

= \(\dfrac{73}{21}\)

   \(\dfrac{1}{5}+\dfrac{5}{35}\)

= \(\dfrac{7}{35}\) + \(\dfrac{5}{35}\)

= \(\dfrac{12}{35}\) 

a)2/3 + 5+3/7:2+4/49.(-7)+1/2.(-2)\(^4\)

= 2/3+5+3/14+-4/7+8

=(2/3+3/14+-4/7)+(5+8)

=13/42+13

=559/42

b)2+3/4.(-0,4)-1-3/5.2,75+(-1/2):4/11

=2+(-0,3) - 1 - 1,65 + (-11/8)

= -2,325

c)( 2 + 1/3 +3 +1/2 ) : ( -4 - 1/6 +3 +1/7 ) +7 +1/2

=35/6 : -43/42 + 7 + 1/2

= -245/43 +7 +1/2

= 155/86

d) 2/3+ -5/12

= 8/12 + -5/12

= 3/12 

e) -2/3 . -9/12 

= \(\frac{-2.-9}{3.12}\)

= 18/36

=1/2

f) -2/9 - 7/12

=\(\frac{-24}{108}\)-  \(\frac{14}{108}\)

= \(\frac{-38}{108}\)

=\(\frac{-19}{54}\)

g) 2/3 : -4/9

= \(\frac{2}{3}\). \(\frac{-9}{4}\)

= -3/2

h) 4 . ( -1/3)\(^3\)- 2 . ( -1/2)\(^2\)+3 .( 1/2 ) +1

=- 4 . 1/27 - 2 . 1/4 + 3 .1/2 +1 

= -4/27 - 1/2 + 3/2 + 1

= 50 / 27

k ) ( 4 - 5/12 ) : 2 +5/24

= 43/12 : 2 +5/24

= 43/24 + 5/24

= 48/24

= 2

\(M=54-\frac{1}{2}.\left(1+2\right)-\frac{1}{3}.\left(1+2+3\right)-....-\frac{1}{12}.\left(1+2+...12\right)\)

\(M=54-\left[\frac{1}{2}.\left(1+2\right)+\frac{1}{3}.\left(1+2+3\right)+....+\frac{1}{12}.\left(1+2+...+12\right)\right]\)

\(M=54-\left(\frac{1}{2}\cdot\frac{2.3}{2}+\frac{1}{3}\cdot\frac{3.4}{2}+....+\frac{1}{12}\cdot\frac{12.13}{2}\right)\)

\(M=54-\left(\frac{3}{2}+\frac{4}{2}+...+\frac{13}{2}\right)=54-44=10\)

Vậy M=10

Bài của bạn đúng rồi!Kb nhe!!

\(C=\dfrac{\sqrt{12}+3}{\sqrt{3}}=2+\sqrt{3}\)