2/5x7+2/7x9+2/9x11+...+2/13x15

=1/5x7+1/7x9+1/9x11+...+1/13x15

=1/5-1/15

=3/15-1/15

=2/15

A = \(\dfrac{2}{5\times7}\)+ \(\dfrac{2}{7\times9}\)+ \(\dfrac{2}{9\times11}\)+.........+ \(\dfrac{2}{13\times15}\)

A = \(\dfrac{1}{5}\)- \(\dfrac{1}{7}\)+ \(\dfrac{1}{7}\) -\(\dfrac{1}{9}\)+ \(\dfrac{1}{9}\) - \(\dfrac{1}{11}\)+ .......\(\dfrac{1}{13}\) - \(\dfrac{1}{15}\)

A = \(\dfrac{1}{5}\) - \(\dfrac{1}{15}\)

A = \(\dfrac{3-1}{15}\)

A = \(\dfrac{2}{15}\)

Đặt VT=A

\(2A=\frac{2}{5.7}+\frac{2}{7.9}+...+\frac{2}{x\left(x+2\right)}\)

\(2A=\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{x}-\frac{1}{x+2}\)

\(A=\left(\frac{1}{5}-\frac{1}{x+2}\right):2\)

Thay A vào VT ta đc:

\(\left(\frac{1}{5}-\frac{1}{x+2}\right):2=\frac{7}{95}\)

\(\frac{1}{5}-\frac{1}{x+2}=\frac{14}{95}\)

\(\frac{1}{x+2}=\frac{1}{19}\)

\(\Rightarrow x+2=19\)

\(\Rightarrow x=17\)

\(\text{Đặt VT=A }\)

\(2A=\frac{2}{5.7}+\frac{2}{7.9}+...+\frac{2}{x\left(x+2\right)}\)

\(2A=\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{x}-\frac{1}{x+2}\)

\(A=\left(\frac{1}{5}-\frac{1}{x+2}\right):2\)

\(\text{Thay A vào VT ta đc: }\)

\(\left(\frac{1}{5}-\frac{1}{x+2}\right):2=\frac{7}{95}\)

\(\frac{1}{5}-\frac{1}{x+2}=\frac{14}{95}\)

\(\frac{1}{x+2}=\frac{1}{19}\)

\(\Rightarrow x+2=19\)

\(\Rightarrow x=17\)

Tích ban đầu là :   \(7\times9=63\)

Tích lúc sau là :     \(63+18=81\)

Ta thấy :  \(81=9\times9\)mà thừa số thứ hai là 9 rồi, vậy 2 đơn vị đó sẽ được thêm vào thừa số thứ nhất
 

a) \(\left(1-\dfrac{1}{3}\right)\times\left(1-\dfrac{2}{5}\right)\times\left(1-\dfrac{2}{7}\right)\times\left(1-\dfrac{2}{9}\right)\)

\(=\left(\dfrac{3}{3}-\dfrac{1}{3}\right)\times\left(\dfrac{5}{5}-\dfrac{2}{5}\right)\times\left(\dfrac{7}{7}-\dfrac{2}{7}\right)\times\left(\dfrac{9}{9}-\dfrac{2}{9}\right)\)

\(=\dfrac{2}{3}\times\dfrac{3}{5}\times\dfrac{5}{7}\times\dfrac{7}{9}\)

\(=\dfrac{2\times3\times5\times7}{3\times5\times7\times9}\)

\(=\dfrac{2}{9}\)

b) \(\dfrac{1}{1\times3}+\dfrac{1}{3\times5}+\dfrac{1}{5\times7}+\dfrac{1}{7\times9}\)

\(=1-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{9}\)

\(=1-\dfrac{1}{9}\)

\(=\dfrac{9}{9}-\dfrac{1}{9}\)

\(=\dfrac{8}{9}\)

Câu1: -12

Câu 2: -15/7

Cau3: -7/9

Caau4: 36/95

10 nhaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

A. 7 x 9 = 63

B. 6 x 4 = 24

C. 4 x 9 = 36

D. 9 x 9 = 81

HT~~

a) (137x25-25x37):10

=25x(137-37):10

=25x100:10

=2500:10

=250

b) 2/7x21/4-2/7:13/4

=2/7x21/4-2/7x4/13

=2/7x(21/4-4/13)

=2/7x257/52

=257/182

c)(124x237+152):(870+235x122)

=(29388+152):(870+28670)

=29540:29540

=1

a) (137 x 25 - 25 x 37 ) : 10 = (25 x (137 - 37) ) : 10

                                       = (25 x 100) : 10

                                       = 2500 : 10

                                       = 250

a)=(3425-925):10

=25200:10

=250

b)=3/2-8/91

=257/182

c)=(29388+152):(870+28670)

=29540:29540

=1

d)=1161+1539/72

=2700/72=75/2

e)=10/77+45/77x9/7

=10/77+405/539

=475/539

li ke cho mình nha Lam Le thi