dau . la dau x

a/ 1.3.2.4.3.5.4.6.5.7/2.2.3.3.4.4.5.5.6.6=1.7/2.6=7/12

b/ ab.aba=abab

        aba=abab:ab

        aba=101

=>a=1     b=0

aabb : ab = 99 hay ab x 99 = aabb hay  ab x100 – ab = aabb

Ta có phép tính

                  __  ab00

                      ___ab___

                        aabb

 b=0 hoặc b=5

Nếu b=0 thì    a000 – a0 = aa00  (sai)

Nếu b=5 thì  

                   __  a500

                        __a5___

                        aa55

            a=4

c) thay a=7/6 b=6/5 thi 3 x a + 4 : b - 5/12=3.7/6+4.6/5-5/12=7/2+24/5-5/12=210/60+288/60-25/60=473/60

**** nha

\(\frac{1.3.2.4.3.5.4.6.5.7}{2.2.3.3.4.4.5.5.6.6}=\frac{\left(2.3.4.5.6\right).\left(3.4.5.7\right)}{\left(2.3.4.5.6\right).\left(2.3.4.5.6\right)}=\frac{7}{12}\)

 chỉ trả lời được câu a thôi: phần a bằng 7/12

x×2+x×3+x×4+x=2013

x×(1+2+3+4)=2013

x×10=2013

⇔x=2013/10 

\(x\times2+x\times3+x\times4+x=2013\)

\(x\times\left(1+2+3+4\right)=2013\)

\(x\times10=2013\)

\(\Leftrightarrow x=\frac{2013}{10}\)

P/s tham khảo nha

X x 2 + X x 3 + X x 4 + X = 2013

X x ( 2 + 3 + 4 + 1 ) = 2013

X x 10 = 2013

X = 2013 : 10

X = \(\frac{2013}{10}\)

\(\frac{1}{1\cdot2}+\frac{1}{2\cdot3}+\frac{1}{3\cdot4}+..+\frac{1}{x\left(x+1\right)}\)

\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{x}-\frac{1}{x+1}\)

\(=1-\frac{1}{x+1}\)

\(=\frac{x+1-1}{x+1}=\frac{x}{x+1}\)

\(\frac{1}{1\times2}+\frac{1}{2\times3}+\frac{1}{3\times4}+...+\frac{1}{x.\left(x+1\right)}\)

\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{x}-\frac{1}{x+1}\)

\(=1-\frac{1}{x+1}\)

\(=\frac{x+1}{x+1}-\frac{1}{x+1}\)

\(=\frac{x}{x+1}\)

\(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+....+\frac{1}{x\left(x+1\right)}\)

\(\Leftrightarrow\frac{1}{1}.\frac{1}{2}+\frac{1}{2}\frac{1}{3}+\frac{1}{3}.\frac{1}{4}+....+\frac{1}{x}\frac{1}{x+1}\)

\(\Leftrightarrow\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+....+\frac{1}{x}-\frac{1}{x+1}\)

\(\Leftrightarrow1-\frac{1}{x+1}\)

rút gọn là được mà ! 

\(\frac{1}{2}\)

\(\frac{7}{14}\)

= 1/2 - 1/3 + 1/3 - 1/4 + ...+ 1/x - 1/x + 1

= 1/2 - 1 /x + 1 =199/200

=1/x+1 = 1/2 - 199/200

1/ x + 1 = ....

\(\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+.....+\frac{1}{x.\left(x+1\right)}=\frac{199}{200}\)

\(\Rightarrow\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+....+\frac{1}{x}-\frac{1}{x+1}=\frac{199}{200}\)

\(\Rightarrow\frac{1}{2}-\frac{1}{x+1}=\frac{199}{200}\)

\(\Rightarrow\frac{1}{x+1}=\frac{1}{2}-\frac{199}{200}\)

\(\Rightarrow\frac{1}{x+1}=-\frac{99}{200}\)

\(\Rightarrow\frac{-99}{-\left(x+1\right).99}=\frac{-99}{200}\)

\(\Rightarrow-99x+-99=200\)

\(x=-1\) nhé.

1-1/x+1=2015/2016

=>1/x+1=1-2015/2016=1/2016

=>x+1=2016=>x=2015

mình không ghi lại đề nha:

\(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{x}-\frac{1}{x+1}=\frac{2015}{2016}\)

<=>\(1-\frac{1}{x+1}=\frac{2015}{2016}\)

<=>\(\frac{x}{x+1}=\frac{2015}{2016}\)

=>x=

Đến đó bạn tự giải tiếp ha

=>(1/1-1/2)+(1/2-1/3)+(1/3-1/4)+...+(1/x-1/x+1)=2015/2016
=>1/1-1/x+1=2015/2016
=>x/x+1=2015/2016
=>x=2015