12 + 12 + 12 + 12 + 12

= 12 x 5

= 60

12+12+12+12+12

= 12x5 

=60

tk nhé

12+12+12+12+12=12.5=60

120

=12 :1/4 + 12x9 -12 : 1/2

=12 x4 + 12x9 -12x2= 12 x( 4+9-2)

= 12 x 11= 132

120

=2.(6/1.4.7 + 6/4.7.10 + 6/7.10.13 + ... + 6/54.57.60)

=2.(1/1.4-1/4.7+1/4.7-1/7.10+1/7.10-1/10.13+...+1/54.57-1/57.60)

=2(1.4-1/57.60)

TỰ TÍNH

  12 x 1 + 12 x 2 + 12 x 3 + 12 x 4

= 12 x (1 + 2 + 3 + 4)

= 12 x 10

= 120

12*1+12*2+12*3+12*4

=12*(1+2+3+4)

=12*10

=120

=120 tích nha bn

Ta gọi vế \(\left(12\cdot2+12\cdot4+12\cdot6\right)\)là vế \(\left(1\right)\)

Ta gọi vế \(\left(2+4+...+12+14\right)\)là vế \(\left(2\right)\)

\(\left(1\right):\)\(12\cdot\left(2+4+6\right)=12\cdot12=144\)

\(\left(2\right):\)\(\frac{\left(14-2\right)}{2}+1=7\)( số )\(=\left(14+2\right)\cdot\frac{7}{2}=28\)

Vậy ta có \(:\)

\(A=\frac{144}{56}=2.5714\)

Đáp số : \(2.5714\)

\(\dfrac{5}{12}+\dfrac{8}{12}+\dfrac{7}{12}+\dfrac{4}{12}=\dfrac{5+8+7+4}{12}=\dfrac{24}{12}=2\)

sai

nhầm đúng

Rút gọn biểu thức cho 3 ta được biểu thức mới là 

\(\frac{4}{28}+\frac{4}{70}+\frac{4}{130}+...\frac{4}{700}\)

= \(\frac{4}{4.7}+\frac{4}{7.10}+\frac{4}{10.13}+...+\frac{4}{25.28}\)

=\(\frac{4}{3}\left(\frac{3}{4.7}+\frac{3}{7.10}+\frac{3}{10.13}+...+\frac{3}{25.28}\right)\)

= \(\frac{4}{3}\left(\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+\frac{1}{10}-\frac{1}{13}+...+\frac{1}{25}-\frac{1}{28}\right)\)

= \(\frac{4}{3}\left(\frac{1}{4}-\frac{1}{28}\right)\)

= \(\frac{4}{3}.\frac{3}{14}\)

= \(\frac{2}{7}\)