5x6x7x9=1890

12x7x27=2268

a,5x6x7x9=1890

b,12x7x27=2268

like cho mình nha :<

\(\frac{42.37}{74.84}=\frac{1554}{3552}\)

\(\frac{4.5.6}{12.10.8}=\frac{1}{8}\)

dấu . là dấu nhân nha

1*1

____=1/4

2*2      

A = \(\dfrac{2}{1\times3\times5}\) + \(\dfrac{2}{3\times5\times7}\) + \(\dfrac{2}{5\times7\times9}\)+\(\dfrac{2}{7\times9\times11}\)

A = \(\dfrac{1}{2}\) x (\(\dfrac{4}{1\times3\times5}\) + \(\dfrac{4}{3\times5\times7}\) + \(\dfrac{4}{5\times7\times9}\) + \(\dfrac{4}{7\times9\times11}\))

A = \(\dfrac{1}{2}\)x (\(\dfrac{1}{1\times3}\)-\(\dfrac{1}{3\times5}\)+\(\dfrac{1}{3\times5}\)-\(\dfrac{1}{5\times7}\)+\(\dfrac{1}{5\times7}\)-\(\dfrac{1}{7\times9}\)+\(\dfrac{1}{7\times9}\)-\(\dfrac{1}{9\times11}\))

A = \(\dfrac{1}{2}\)x (\(\dfrac{1}{1\times3}\) - \(\dfrac{1}{9\times11}\))

A = \(\dfrac{1}{2}\) x (\(\dfrac{1}{3}-\dfrac{1}{99}\))

A = \(\dfrac{1}{2}\times\) \(\dfrac{32}{99}\)

A = \(\dfrac{16}{99}\)

B = \(\dfrac{1}{1\times2\times3}\) + \(\dfrac{1}{2\times3\times4}\) + \(\dfrac{1}{3\times4\times5}\) + \(\dfrac{1}{4\times5\times6}\)

B = \(\dfrac{1}{2}\) x (\(\dfrac{2}{1\times2\times3}+\dfrac{2}{2\times3\times4}+\dfrac{2}{3\times4\times5}+\dfrac{2}{4\times5\times6}\))

B = \(\dfrac{1}{2}\) x (\(\dfrac{1}{1\times2}\)-\(\dfrac{1}{2\times3}\) + \(\dfrac{1}{2\times3}\)-\(\dfrac{1}{3\times4}\)+\(\dfrac{1}{3\times4}\)-\(\dfrac{1}{4\times5}\)+\(\dfrac{1}{4\times5}\)-\(\dfrac{1}{5\times6}\))

B = \(\dfrac{1}{2}\)x(\(\dfrac{1}{1\times2}\) - \(\dfrac{1}{5\times6}\))

B = \(\dfrac{1}{2}\)x (\(\dfrac{1}{2}-\dfrac{1}{30}\))

B = \(\dfrac{1}{2}\)x \(\dfrac{7}{15}\)

B = \(\dfrac{7}{30}\)

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

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

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

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

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

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

ta có : B = 1.2.3 + 2.3.4 + 3.4.5 + ...... + 2016.2017.2018

4B = 1.2.3.4 - 1.2.3.4 + 2.3.4.5 - 2.3.4.5 + ...... + 2016.2017.2018.2019

4B = 2016.2017.2018.2019

vậy B = 2016.2017.2018.2019/4 

Ta có : B = 1.2.3 + 2.3.4 + ...... + 2016.2017.2018

=> 4B = 1.2.3.4 - 1.2.3.4 + 2.3.4.5 - 2.3.4.5 + ...... + 2016.2017.2018.2019

=> 4B = 2016.2017.2018.2019

=> B = 2016.2017.2018.2019/4

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

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

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

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

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

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

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

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

120/960 nhé

ai k cho mình tròn 70 với