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A. 7 x 9 = 63

B. 6 x 4 = 24

C. 4 x 9 = 36

D. 9 x 9 = 81

HT~~

=7+7x9 +7x90

=7x(1+9+90)

= 7 x 100

=700

=7+(7x9+7)x90

=7+70x90

=7+6300

=6307

        \(\dfrac{2}{7}\) \(\times\) \(\dfrac{9}{7}\) - \(\dfrac{18}{49}\) \(\times\) \(\dfrac{1}{2}\)

    =  \(\dfrac{18}{49}\)  - \(\dfrac{18}{49}\) \(\times\) \(\dfrac{1}{2}\)

    =   \(\dfrac{18}{49}\)  \(\times\) 1 - \(\dfrac{18}{49}\) \(\times\) \(\dfrac{1}{2}\)

    =    \(\dfrac{18}{49}\) \(\times\) ( 1 - \(\dfrac{1}{2}\) )

    =      \(\dfrac{18}{49}\) \(\times\) \(\dfrac{1}{2}\)

    =          \(\dfrac{18}{98}\)

    =           \(\dfrac{9}{49}\)

Gọi biểu thức đó là S nha.

S= 336/505

336/505

7/4 x 15/7 + 13/7 x 9/4 -7/4 x 2/7

=7/4 x ( 15/7- 2/7) + 13/7 x 9/4

=7/4 x 13/7 + 13/7 x 9/4

=13/7 x (7/4 x 9/4)

=13/7 x 16/4

=13/7 x 4

=52/7

7/4 x 15/7 + 13/7 x 9/4 -7/4 x 2/7

=7/4 x ( 15/7- 2/7) + 13/7 x 9/4

=7/4 x 13/7 + 13/7 x 9/4

=13/7 x (7/4 + 9/4)

=13/7 x 16/4

=13/7 x 4

=52/7

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:

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

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

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

\(\Rightarrow x+2=19\)

\(\Rightarrow x=17\)

\(\text{Đặt VT=A }\)

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

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

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

\(\text{Thay A vào VT ta đc: }\)

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

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

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

\(\Rightarrow x+2=19\)

\(\Rightarrow x=17\)

27/182-1/182-1/7

26/182-1/7

1/7-1/7

0

Đề bài là gì bạn ơi ??

de bai la tinh

\(C=\frac{3}{7}\times\frac{4}{13}+\frac{3}{7}\times\frac{9}{13}+5\frac{4}{7}\)

\(C=\frac{3}{7}\times\frac{4}{13}+\frac{3}{7}\times\frac{9}{13}+\frac{39}{7}\)

\(C=\frac{3}{7}\times\left(\frac{4}{13}+\frac{9}{13}\right)+\frac{39}{7}\)

\(C=\frac{3}{7}\times1+\frac{39}{7}\)

\(C=\frac{3}{7}+\frac{39}{7}\)

\(C=\frac{42}{7}\)

\(C=6\)

_Chúc bạn học tốt_