TL

=-3545807085

nha bn chưa chưa chắc đã đúng

HT 

=6,953,656,803,105

TL:

\(=-3545807085\)

Phải dùng đến máy tính

_HT_

 \(\frac{1}{3}+\frac{1}{6}+\frac{1}{10}+...+\frac{1}{\frac{x\times\left(x+1\right)}{2}}=\frac{2015}{2017}\)

\(\Leftrightarrow\frac{1}{2}\)[\(\frac{1}{3}+\frac{1}{6}+\frac{1}{10}+...+\frac{1}{\frac{x\times\left(x+1\right)}{2}}\)]=\(\frac{1}{2}.\frac{2015}{2017}\)

\(\Leftrightarrow\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{x\times\left(x+1\right)}=\frac{2015}{4034}\)

\(\Leftrightarrow\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{x}-\frac{1}{x+1}=\frac{2015}{4034}\)

\(\Leftrightarrow\frac{1}{2}-\frac{1}{n+1}=\frac{2015}{4034}\)\(\Leftrightarrow\frac{1}{x+1}=\frac{1}{2}-\frac{2015}{4034}=\frac{1}{2017}\)<=> x+1=2017<=>x=2016

quá khó

Trương Thừa Long chuẩn

Đề khó làm không được

:))

2 + 1 + 3 + 5 = 3 + 6 + x

      11 = 9 + x

      11 = 9 + 2

x = 2

2+1+3+5=3+6+.......

*2+1+3+5=3+6+1+1

^_^

2+1+3+5=3+6+2

  1/2 + 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + 1/56 + 1/72 + 1/90
=1/(1.2)+1/(2.3)+1/(3.4)+1/(4.5)
+1/(5.6)+1/(6.7)+1/(7.8)
+1/(8.9)+1/(9.10)
=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5.+1/5-1/6...
+1/9-1/10
=1-1/10
=9/10

\(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+...+\frac{1}{9.10}\)

\(=\frac{1}{1x2}+\frac{1}{2x3}+\frac{1}{3x4}+...+\frac{1}{9x10}\)

\(=\frac{2-1}{1x2}+\frac{3-2}{2x3}+\frac{4-3}{3x4}+...+\frac{10-9}{9x10}\)

\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{9}-\frac{1}{10}\)

\(=1-\frac{1}{10}\)

\(=\frac{10}{10}-\frac{1}{10}\)

\(=\frac{9}{10}\)

Tham khảo nha !!! 

\(\frac{1}{2}\)+ \(\frac{1}{6}\)+ \(\frac{1}{12}\)+ \(\frac{1}{20}\)+ \(\frac{1}{30}\)+ \(\frac{1}{42}\)

= \(\frac{6}{7}\)

\(\frac{1}{1\cdot2}\) +\(\frac{1}{2\cdot3}\) +\(\frac{1}{3\cdot4}\) +\(\frac{1}{4\cdot5}\) +\(\frac{1}{5\cdot6}\) +\(\frac{1}{6\cdot7}\)                                                                                                               =1-\(\frac{1}{7}\) =\(\frac{6}{7}\)

\(=\frac{6}{7}\)

=2

=4

kb nha

1+1 =2 

2+2=4

1 + 1 = 2

 2 + 2 = 4

5.36536796537

cách làm như thế nào

\(1+2+3+4+5+6+7+8+9\)\(=45\)

1+2+3+4+5+6+7+8+9=45.
tớ nhanh k nha.

1 + 2 + 3 + 4 + 5 + 6 + 7 +8 + 9 =  45 

bạn ạ , nhớ k cho mình nhé !