TA có: \(\frac{4\times5+6}{4\times5\times6}\)

\(=\frac{20+6}{20\times6}=\frac{26}{120}=\frac{13}{60}\)

`(4xx5xx6)/(12xx8xx10)=(4xx5xx6)/(6xx2xx4xx2xx5xx2)= 1/(2xx2xx2)=1/8`

120/960 nhé

4 x 5 x 6

= 20 x 6 

= 120

12 x 8 x 88

= 96 x 88

= 8448

Chúc bạn may mắn nah!!!

4 x 5 x 6

= 20 x 6

= 120

12 x 8 x 88

= 96 x 88

= 8448

\(A=1\cdot2\cdot3+2\cdot3\cdot4+\cdots+n\left(n+1\right)\left(n+2\right)\)

\(=2\left(2-1\right)\left(2+1\right)+3\left(3-1\right)\left(3+1\right)+\cdots+\left(n+1\right)\left(n+1-1\right)\left(n+1+1\right)\)

\(=2\left(2^2-1\right)+3\left(3^2-1\right)+\cdots+\left(n+1\right)\left\lbrack\left(n+1\right)^2-1\right\rbrack\)

\(=\left\lbrack2^3+3^3+\cdots+\left(n+1\right)^3\right\rbrack-\left(2+3+\cdots+n+1\right)\)

\(=\left\lbrack1^3+2^3+\cdots+\left(n+1\right)^3\right\rbrack-\left(1+2+\cdots+n+1\right)\)

\(=\left(1+2+\cdots+n+1\right)^2-\left(1+2+\cdots+n+1\right)\)

\(=\left\lbrack\frac{\left(n+1\right)\left(n+2\right)}{2}\right\rbrack^2-\frac{\left(n+1\right)\left(n+2\right)}{2}=\frac{\left(n+1\right)^2\cdot\left(n+2\right)^2-2\left(n+1\right)\left(n+2\right)}{4}\)

5*6*7*9/12*7*27

=5*6*7*3*2/2*6*7*3*3

=5*1*1*1*1/1*1*1*1*3

=5/3

5x6x7x9 / 12x7x27 = 5x6x7x9 / 2x6x7x3x9 = 5 / 2x3 = 5 / 6

4x5x6 / 12x10x8+1 = 4x5x6 / 2x6x2x5x2x4+1 = 1 / 2x2x2+1 = 1 / 8+1 = 1 / 9

\(\frac{4\times5\times6}{12\times15\times9}\)\(=\frac{2\times2\times5\times6}{2\times6\times3\times5\times9}\)\(=\frac{2}{3\times9}\)\(=\frac{2}{27}\)

\(\frac{6\times8\times11}{33\times16}\)\(=\frac{2\times3\times8\times11}{3\times11\times8\times2}\)\(=\)\(1\)

~ HOK TỐT ~

4×5×612×15×912×15×94×5×6​=2×2×5×62×6×3×5×9=2×6×3×5×92×2×5×6​=23×9=3×92​=227=272​

6×8×1133×1633×166×8×11​=2×3×8×113×11×8×2=3×11×8×22×3×8×11​$=$$1$

a. \(\frac{3.5.7}{6.21}=\frac{21.5}{21.6}=\frac{5}{6}\)

b. \(\frac{2.3.5}{4.5.6}=\frac{6.5}{4.5.6}=\frac{1}{4}\)

\(\frac{2}{1.2.3}+\frac{2}{2.3.4}+...+\frac{2}{6.7.8}\)

\(=\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+...+\frac{1}{6.7}-\frac{1}{7.8}\)

\(=\frac{1}{1.2}-\frac{1}{7.8}\)

\(=\frac{1}{2}-\frac{1}{56}\)

\(=\frac{28}{56}-\frac{1}{56}=\frac{27}{56}\)

Dấu . là nhân nha

\(\frac{2}{1.2.3}=\frac{1}{1.2}-\frac{1}{2.3}\)

\(\frac{2}{2.3.4}=\frac{1}{2.3}-\frac{1}{3.4}\)

.......................................

\(\frac{2}{6.7.8}=\frac{1}{6.7}-\frac{1}{7.8}\)

S= \(\frac{1}{1.2}-\frac{1}{7.8}=\frac{27}{56}\)